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Full text of "Solubilization and mobilization of perchloroethylene by cosolvents in porous media"

SOLUBILIZATION AND MOBILIZATION OF PERCHLOROETHYLENE BY 
COSOLVENTS IN POROUS MEDIA 



By 
MICHAEL EDWARD VAN VALKENBURG 



A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL 

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT 

OF THE REQUIREMENTS FOR THE DEGREE OF 

DOCTOR OF PHILOSOPHY 

UNIVERSITY OF FLORIDA 

; ' 1999 



This dissertation is dedicated to each person who gave his or her life so valiantly and 
courageously for our country. I know this is but a little gesture, but nonetheless their 
sacrifices have moved me so deeply, that any acknowledgement of them in remembrance is 
the least that each of us can do. I hope in just reading this, you will remember them. 



ACKNOWLEDGMENTS J; 

I would certainly not be in the position to be even writing these acknowledgements 
if it were not for the support of my family throughout the years. Through several times of 
self-evaluation and self re-direction, they have always been supportive and helpful, 
providing motivation when I had very little - especially my Mom. I am thankful for my 
Dad, for "buying me books and teaching me all he knew." I know he was far from 
perfect, so am I, but he was, is, and always will be my Dad. 

My wife, Kim, and our three children, Joseph, Lauryn, and Kelley have been so 
imderstanding about times devoted away from them when they have wanted me the most, 
and my attentiveness that could have always been better. There was always something 
else on my mind - to finish this! I wish I could have been there even more. I will always 
try to be a better father and husband. There is more to come. 

I extend my sincere appreciation to the U.S. Air Force (and the American 
taxpayer) for sponsoring my education and completion of this dissertation. I thank the 
Biomedical Sciences Corps for its flexibility and thank the U.S. Air Force Academy and 
the Department of Chemistry for having the faith in me to complete this degree and 
wanting me back to work in what has to be one of the nicest environments in the world! 
Special thanks go to Col. Hans Mueh, Col. Clifford Utermoehlen, Lt. Col. Ron Furstenau, 
and Maj. Rob Racicot. 



m 



Obvious thanks go to Dr. Mike Annable and my committee members for their 
guidance in my completion of this project. Special thanks go to Dr. Joe Delfino for taking 
the time to sit down for lunch with me in a deli in Denver, Colorado during an ACS 
convention and encouraging me to become a Gator! Also, I valued his graduate program 
guidance and wise advice that I took on numerous occasions. Thanks are given to Dr. Bill 
Wise for his many hours of exchanging knowledge, in the lab and in the office, and to Dr. 
Suresh Rao for his confidence building. Finally, special gratitude to Mike Annable for 
being so understanding, supportive, and always open to my impromptu oflSce visits 
seeking guidance and tutelage. 

Thanks also go to my fellow lab partners, Michael Brooks, Jaehyun Cho, Clayton 
Clark, and Rick Young for their many instances of assistance and showing patience with 
me, and to Randy Switt for all his computer help. To my fellow 2-D boxer and final lab 
inmate - Jim Jawitz - 1 appreciate the use of "the box," his motivational assistance, the 
hours of practical knowledge, and the many laughs. He will not be "forgotten" in this 
document. 

One final person, who I am sure does not get recognized enough and whose 
expertise I am gratefijl for, is Lynn LaBauve, of the Marston Science Library Reference 
Desk. Over the last three years she has sought me out numerous times to help in 
searching the vast reference databases used in completing research. Those needing 
reference help at the University of Florida should find her and they will reap the benefits. 



IV 



TABLE OF CONTENTS 

page 

ACKNOWLEDGMENTS iii 

LIST OF TABLES viu 

LIST OF FIGURES ix 

ABSTRACT xiii 

CHAPTERS 

L INTRODUCTION 1 

Background 1 

Selection of DNAPL 5 

Study Objectives 5 

Dissertation Organization 8 

2. INVESTIGATIONS OF THE RELATIONSHIP OF COSOLVENT 
FRACTION TO PERCHLOROETHYLENE (PCE) SOLUBILITY AND 
EQUILIBRIUM INTERFACIAL TENSION 9/ 

Introduction 9 

Comparison of the Molecular Structures of Water and Low Molecular Weight 

Alcohols 10 

Solubility of Hydrocarbons into Water/alcohol Mixtures and its Relationship to 

Amount of Cosolvent 13 

Log-linear relationship 15 

Cosolvency power 17 

Other methods for solubility estimation 17 

Choice ofsolubility estimation method 21 

Interfacial Tension of Ternary Alcohol/water/PMOS Mixtures 21 

Relation to amount of cosolvent 22 

Different interfacial tension prediction methods 23 

Relation of IFT to Solubility of Organic Solute 27 

Materials and Methods 27 

Results and Discussion 29 

Log Linear Solubility Estimation 29 

Ethanol 29 



Isopropanol (IPA) 30 

UNIFAC Method 31 

Extended Hildebrand Method 31 

Minor cosolvent addition 35 

Interfacial Tension Measurements and Predictions 36 

Conclusions 39 

3. MOBILIZATION OF RESIDUAL PERCHLOROETHYLENE DURING 
COSOLVENT FLOODING 42 

Introduction 42 

Solubilization vs. Mobilization 45 

The Trapping Number Relationship 47 

Study Objective 49 

Materials and Methods 50 

GC Analysis 50 

Physical Measurements 51 

Sand Column Preparation 51 

PCE Saturation 52 

Hydrodynamic Parameters 52 

Sand Column Mobilization Studies 53 

Results and Discussion 54 

Equilibrated Gradient Column Studies 54 

Blank Equilibrated Gradient Study 55 

Non Equilibrated Column Studies 55 

Generation of Mobilization Curves 58 

Swelling Effects of Cosolvents 62 

Conclusions 64 

4. ENTRAPMENT VERSUS MOBILIZATION OF RESIDUAL 
PERCHLOROETHYLENE DURING COSOLVENT FLOODING 66 

Introduction 66 

Solubilization, Mobilization and the Trapping Number Relationship 68 

Mobilization and Entrapment of Residual Non-Aqueous Phase Liquid 68 

Study Objective 70 

Materials and Methods 70 

Physical Measurements 71 

Sand Column Preparation 71 

PCE Saturation and Generation of Trapping Curves 72 

Mobilization studies 72 

Entrapment studies 74 

Hydrologic Parameters 74 

Results and Discussion 76 

Entrapment in Homogeneous Sand Column 77 

Effect of Pore Size Heterogeneity on the Entrapment of PCE 80 



VI 



Conclusions 83 

5. MOBILIZATION AND ENTRY OF DNAPL POOLS INTO FINER SAND 
MEDIA: TWO-DIMENSIONAL BOX STUDIES 85 

Introduction 85 

Materials and Methods 90 

General Packing Procedure 91 

Dye Tracer Displacement 93 

DNAPL Introduction 94 

Hydraulic Controls During 2-D Box Experiments 94 

Results and Discussion 95 

No. 100-140 Fine Layer 95 

Step input of 100% alcohol 95 

One-dimensional horizontal sand column experiments 98 

Step input of 80% alcohol 99 

No. 60-70 Fine Layer 101 

Step input of 80% alcohol 101 

Gradient Injection (10-90%) of Alcohol 101 

No. 40-50 Fine Layer 105 

Background dye flush after DNAPL injection 105 

Step input of 80% alcohol 106 

No. 30-40 Fine Layer 107 

Step input of 80% alcohol 107 

Step input of 70% alcohol 109 

Step input of 50% alcohol 110 

Two-Dimensional Studies witht-Butyl Alcohol 110 

Step input of 30% TBA: #30-40 finer layer Ill 

Step input of 40% TBA: #100-140 finer layer Ill 

Systematic Quantitative Evaluation and Prediction of Mobilization into Finer 

Layers 116 

Conclusions 120 

6. SUMMARY AND CONCLUSIONS 122 

APPENDICES 

A MOISTURE RELEASE CURVES FOR SAND MEDIA 127 

B TWO-DIMENSIONAL BOX SCHEMATICS 134 

REFERENCES 147 

BIOGRAPHICAL SKETCH 156 



vn 



LIST OF TABLES 

Table page 

2-L Solubility parameters for study components (Barton 1975) 32 

3-1. Physical Measurements of PCE Saturated Cosolvent Solutions 57 

3-2. Physical properties of solutions used in swelling mobilization studies 64 

4-1. Results oflinear regression of entrapment studies 79 

5-1. Particle size ranges of sands used 92 

5-2. Summary of Experimental Runs in 2-Dimensional Box Studies 96 

5-3 - Summary of desaturation profile curve fitting parameters. Beit Netofa Clay values 
(a, n, and m) are fi-om van Genuchten (1980). Pore radius for the clay is taken fi-om 
Wise (1992) 117 

5-4 - Results of globule force balance calculations. Mobilization of globule is predicted if 
hdnapi > h'J ' '^' . PermeabiUty of 20-30 medium measured to be 6.35E-7 cml "Clay" 

scenario based on Beit Netofa clay (van Genuchten 1980) is shown for comparison. 
Fluid property values shown are approximate and for illustrative purposes 119 



vm 



LIST OF FIGURES 



"'-*. 



Figure '■ . ^ page 

2-1. Graph of data from Franks and Ives (1966), relating concentration of hydrogen bonds 
to volume fraction of ethanol 12 

2-2. Solubility of PCE as a function of cosolvent volume fractions (initial phase 

volumetric phase ratio 1:1) 30 

2-3. Comparison of Measured Solubility Data and those predicted by the UNIFAC 

Method 32 

2-4. PCE Solubility Prediction of the Hildebrand and Extended-Hildebrand Theories for 
the IPA Cosolvent Mixtures 34 

2-5. Solubility of PCE as a fianction of various cosolvent volume fractions (initial phase 
volumetric phase ratio 1:1) 36 

2-6. Relationship of equilibrated interfacial tension of PCE/alcohol/water ternary systems 
as a fimction of initial cosolvent volume fraction 37 

2-7. Logarithmic plot of the IFT of ternary PCE/cosolvent/water mixtures versus initial 
volume fraction of cosolvent. Additional data for addition of 10% isobutanol is 
shown for reference 38 

2-8. Interfacial tension of PCE/cosolvent/water mixtures related to solubility of PCE in 
the aqueous phase. Numbers above selected data points indicate initial volume 
fraction of cosolvent 40 

3-1 . Gradient effluent profile for saturated PCE nm (influent %'s shown are ethanol 

volume fractions prior to saturation) 56 

3-2. Gradient effluent profile using unsaturated ethanol mixtures - percent of mobilization 
shown 56 

3-3. Gradient effluent profile using unsaturated ethanol cosolvent mixtures with PCE 

saturation reduction shown 57 



IX 



3-4. Mobilization curves showing effect of a cosolvent (ethanol) flushing phase which is 
pre-equilibrated with PCE (gradient 2 and 4) and a flushing phase with full 
solubilization potential (non-equilibrated 1). All are gradient runs 59 

3-5. PCE Desaturation Curves - PCE saturated ethanol cosolvent runs compared with 
data from Pennell et al. (1996) 60 

3-6 Ethanol mobilization curves with surfactant run superimposed 61 

3-7. Results from mobilization studies using pre-equilibrated IPA solutions, superimposed 
on the ethanol study results 63 

3-8. Results of mobilization of PCE during gradient TBA column flushing; TBA pre- 
equilibrated with PCE 63 

4-1. Moisture release curve for No. 30-40 silica sand used for these studies, conducted 
via Tempe cell, van Genuchten (1980) and Brooks & Corey (1964) fits are based on 
minimizing the sum of squares of the difference between the actual data and the fitted 
line 75 

4-2. Relative permeability to the wetting phase at less than normal nonwetting phase 

residual saturations: Morrow and Songkran (1982) data shown with regression (R = 
0.907) and fit of this study's Tempe cell data based on van Genuchten (1980) 
parameters and the Mualem (1976) method 76 

4-3. PCE Desaturation curve - experimental ethanol data only compared to those of 

PenneU et al. (1996) 77 

4-4. PCE desaturation curves for both mobilization and entrapment studies, with linear 
regressions shown for the entrapment experiments 79 

4-5. Effective saturation of study 30-40 mesh sand as a fiinction of capillary pressure, 
resulting slope of regressed line is the Brooks and Corey lambda, A, = 3.65 81 

4-6. Results of entrapment experiments on the heterogeneous packing (#20-100 sand), 
shown with homogeneous entrapment and mobilization results for reference 82 

5-1. Schematic of DNAPL contamination of subsurface aquifer systems, showing free 
phase and residual DNAPL 87 

5-2. Typical 2-D box setup after injection of PCE, prior to any flushing 93 

5-3. Dyed PCE injected into Number 20-30 mediimi (approximately 2.7 ml) pooled over 
a 1 cm layer of Number 100-140 medium. 97 



5-4. Removal of residual dyed PCE by gradient ethanol injection (0-100% v/v) over one 
pore volume. Darker band at interface is highly saturated PCE which is mobilizing 
toward the lower left and pooling 99 

5-5. Progression of DNAPL pool collapse - Nos. 20-30 background medium, Nos. 100- 
140 finer layer - after 0.8 PV of 80% v/v ethanol/water step input. Downstream 
direction is to the right in all pictures 100 

5-6. Spreading of DNAPL pool downstream on top of finer Nos 100-140 layer. No 
breakthrough occurred during this run - 1 . 1 PV after 80% v/v ethanol/water step 
input 100 

5-7. Schematic of step input of 80% alcohol - No. 60-70 finer layer 103 

5-8. Horizontal spreading of PCE pool upstream fi-om injection zone - Nos. 20-30 

background media, 60-70 finer layer, 1.1 PV after gradient injection of 10 - 90% v/v 
ethanol/water over IPV. Blue band is location of 58% (leading edge) to 76% 
ethanol 104 

5-9. Highly concentrated cosolvent phase in which dye has partitioned, entering finer 
Nos. 60-70 layer. This is not fi-ee phase mobilization. Blue band above is fi-om a 
post gradient step input to 100% reagent alcohol 104 

5-10 - Breakthrough of highly concentrated PCE containing cosolvent phase into finer 
layer and subsequent reestablishment of DNAPL below the finer layer due to lower 
alcohol concentrations 105 

5-11. Water tracer study for 40-50 finer layer experiment. Note the significant holdup of 
tracer in lower portions of PCE pool and noticeable progression of dye in finer layer 
underneath 106 

5-12. Collapsing of PCE pool and spreading of DNAPL along 40-50 layer. No 

breakthrough of DNAPL observed 107 

5-13. Schematic for step input of 80% alcohol - No. 30-40 finer layer 109 

5-14. Schematic for step input of 70% alcohol - No. 30-40 finer layer 109 

5-15. Mobilization of DNAPL into finer 30-40 layer at two locations, upstream fi-om 
injection zone (small + mark in picture) - 0.45 PV after step input of 80% v/v 
ethanol/water mixture. Mobilization also occurred later at one other location 
downstream of pool (see text) 109 



XI 



5-16. Mobilization of the PCE pool by a 40% v/v TBA cosolvent mixture (0.6 PV) 
resulting in the trapping of a volume of the cosolvent mixture on top of the finer 
layer 113 

5-17. PCE and TBA elution profiles fi-om 2D Box after a step input of 40% v/v 

TBA/H2O. TBA profile data are shown as GC peak areas for reference 1 14 

5-18. DNAPL pool shape after the injection of one pore volume of 40% v/v TBA 

cosolvent mixture 115 

A-1. Moisture release curve for Nos. 20-30 sand 128 

A-2. Pore size fi^equency distribution of Nos. 20-30 sand 128 

A-3. Moisture release curve for Nos. 30-40 sand 129 

A-4. Pore size fi-equency distribution of Nos. 30-40 sand 129 

A-5. Moisture release curve for Nos. 40-50 sand 130 

A-6. Pore size fi"equency distribution of Nos. 40-50 sand 130 

A-7. Moisture release curve for Nos. 60-70 sand 131 

A-8. Pore size fi-equency distribution of Nos. 60-70 sand 131 

A-9. Moisture release curve for Nos. 100-140 sand 132 

A-10. Pore size distribution of Nos. 100-140 sand 132 

A-6-1 1. Moisture release curve for wide distribution (#20-100) sand 133 

A-12. Pore size distribution of wide distribution (#20-100) sand 133 



xu 



Abstract of Dissertation Presented to the Graduate School 
of the University of Florida in Partial Fulfillment of the 
Requirements for the Degree of Doctor of Philosophy 

SOLUBILIZATION AND MOBILIZATION OF PERCHLOROETHYLENE BY 

COSOLVENTS IN POROUS MEDIA 

By 

Michael E. Van Valkenburg 

May 1999 

Chairman: Dr. Michael D. Annable 

Major Department: Environmental Engineering Sciences 

Batch equilibrium studies conducted for perchloroethylene (PCE)/cosolvent 

systems determined that the log-linear solubility relationship is not a completely accurate 

method to predict solubility of PCE in cosolvent mixtures over an entire range of volume 

fi-actions. Batch studies resulted in cosolvency powers of 3.73 and 4.13 for ethanol and 

isopropanol, respectively. However, log-linear predictions may be adequate for 

estimations necessary for remediation efforts. The use of the Extended Hildebrand model 

is recommended. ' : ' . ' 

The interfacial tension (IFT) resulting fi-om cosolvent mixtures when compared to 

the initial volume fi-action of cosolvent showed a relationship, similar to the log-linear 

model. An "IFT reduction power" was determined for ethanol to be -3.60, and isopropyl 

alcohol, -5.80, describing the ability of cosolvents to reduce IFT with increasing volume 



xm 



fraction. IFT values are accurately estimated by PCE solubility in regimes conducive to 
cosolvent flushing. 

Onset of residual PCE mobilization was found to begin at a trapping number (M) 
of 2 X 10"*. Solubilization of residual PCE is dominant at ethanol volume fractions less 
than 85% and mobilization of PCE is avoided. Under severe conditions, mobilization via 
cosolvents can occur. These include large step inputs of high cosolvent fractions (greater 
than 85%), when DNAPL satiiration is great enough for IFT reduction to cause 
mobilization. Behavior of surfactant and cosolvent systems was similar on a mobilization 
curve and is independent of alcohol type. 

Entrapment and mobilization of residual NAPL are separate and distinct processes. 
The entrapment process appeared to be log-linearly related to the trapping nvimber for 
homogeneous media. This is believed to be associated with the log-linear dependence of 
saturation with capillary pressure. However, for heterogeneous media, increased 
saturations with decreasing IFTs was observed. 

Two-dimensional studies revealed that pooled DNAPL was found to collapse 
under reducing IFT conditions and mobilized downward and up gradient along overriding 
cosolvent fronts. This caused significant build-up of DNAPL on the lower confining layer. 
The most significant production of DNAPL through any fine layer in these studies was 
actually up stream from the source zone. Gradient injection to remove pooled DNAPLs 
did not appear to provide significant benefit over step inputs. Entry pressure calculations 
predicted breakthrough of PCE into the finer media in excellent fashion. Breakthrough of 
PCE under typical ethanol flooding conditions (80% by volume) can generally be assumed 
to occur in homogeneous sand media when the cosolvent/DNAPL entry pressure of the 



xiv 



finer media ( h'^"^p' ) is less than 0.35 cm. A swelling alcohol (t-butanol) used to remove 
pooled DNAPL resulted in trapped cosolvent zones on top of finer layer due to density 
effects. Partitioning of TBA into DNAPL allowed for more accumulation on finer layer 
before entry was observed. Calculations for an example clay estimated that approximately 
a half a meter worth of equilibrated PCE-type DNAPL would have to accumulate before 
entry into the clay pores under extreme cosolvent flooding conditions. 



■ f ■ ^\ „ y . :;{ ^■ 



■<&.. 



XV 



CHAPTER 1 
INTRODUCTION 



Background 



Due to our industrial society's ever-increasing use of chemicals during the last 50 
years, it has been increasingly necessary to manage the corresponding waste products 
from these industrial operations. The management of these waste streams at various times 
throughout this half-century has evolved from "drum it up and bury it in the back 40" type 
methods to highly regulated disposal and stream reduction. Unfortunately, prior to the 
1980's, industry did not realize the environmental and health impacts of our decisions, 
which we thought were proper at the time. As a resuh, there are hundreds of thousands of 
disposal sites in the United States alone, thousands of which are severe enough to be on 
the Environmental Protection Agency's (EPA) National Priority List. A large number of 
these sites are contaminated with a class of chemicals known as dense nonaqueous phase 
liquids (DNAPLs), some of which are known carcinogens. These chemicals, immiscible 
with water, include polychlorinated biphenyls (PCBs), creosotes, and halogenated 
solvents. Prior to the early 1990s, this class of contaminants received minimal attention 
from environmental engineers and hydrogeologists. 

Until recently, remediation technologies for the removal of these DNAPLs from 
subsurface environments focused on pumping of groimdwater and subsequent treatment of 
this stream. Risk reduction to possible receptors was the driving force behind these 



actions. However, due to the solubility limitations of these types of treatment, remedial 
action time-scales were long and expensive. The source of contamination is very slowly 
removed due to solubilization into water. In the last few years, research efforts and 
technology demonstrations have become more focused on source removal. These include 
surfectant flooding and cosolvent flushing (Annable et al. 1996; Falta et al. 1997; Fortin et 
al. 1997; Jawitz et al. 1998b; Lunn and Kueper 1997; Pennell and Abriola 1996; Pennell et 
al. 1994; Pope and Wade 1995; Rao et al. 1997). Although these techniques tend to be 
more aggressive and have high initial costs, the removal of a possible long-term source is 
beneficial fi-om risk reduction, economic, and legal perspectives. 

The major concern with the use of siufactants and cosolvents is the possibility of 
DNAPL movement during these remediation operations. The natural driving force behind 
any movement of DNAPL in the subsurface is gravity. Downward DNAPL movement of 
any kind is undesirable as this increases the likelihood of the contaminant leaving the more 
accessible and shallower geologic zones and entering deeper drinking water aquifers. 
Furthermore, once collected on top of a finer layer, entry, and subsequent breakthrough 
presents severe risks to aquifers below. Remediation techniques using surfactant and/or 
cosolvents increase solubility of the DNAPL into the aqueous phase, but concurrently 
increase the possibility of DNAPL movement due to a decrease in interfacial forces 
between the DNAPL and the aqueous phase. In fact, some remediation techniques using 
surfactants have as their main objective the bulk movement of the DNAPL towards 
recovery wells for extraction. This technology has been modified from the enhanced oil 
recovery (EOR) field, where surfactants are used to move oil previously trapped in 



reservoir rock. The movement of any non-aqueous phase liquid in the subsurface has been 
labeled 'mobilization' and will be hereafter referred to as such. 

The use of alcohols to enhance recovery of oils or NAPLs via miscible 
displacement has long received attention (Morse 1952). Several other references to 
alcohol use appear in early literature on the topic, including Gatlin (1959), Gatlin and 
Slobod (1960), Kamath (1960), Paulsell (1953), Sievert (1958), and Slobod et al. (1958). 

Due to the inherent rislcs of downward mobilization of DNAPLs, the use of 
cosolvents to remove them via miscible displacement has increased in popularity. This is 
due to the primary objective of 'cosolvent flushing' being solubilization of the DNAPL, 
rather than mobilization. However, this is not to say that mobilization does not occur if 
cosolvents are used. Since the solubility of a contaminant increases generally 
logarithmically with addition of a cosolvent to water, use of cosolvents (such as ethanol or 
isopropyl alcohol) in their pure state, or at least at high volume fractions, would appear to 
be a consistently wise choice. Nevertheless, mobilization of DNAPL at these high volume 
fractions (>80% by volimie) of cosolvents is very possible, especially if DNAPL 
saturations are above residual levels. 

DNAPLs in the subsurface can also be residually trapped in the vadose and 
phreatic zones of the subsurface. Here the DNAPL exists as discrete globules in the pore 
of the soil mediimi. Eventually, the draining DNAPL in the saturated zone can become 
'pooled' on top of a layer of soil that is more restrictive to downward flow of any fluid. It 
is less permeable than the surrounding layers. Here, it can spread laterally along the less- 
permeable layer until equilibrium is achieved. Alternatively, continuing quantities of 



■:^' 



DNAPL can accumulate and the height of the pool becomes great enough to where 
gravity forces the DNAPL to enter the smaller pores of the less-permeable layer. 

Mobilization of DNAPL can occur during the remediation of both these types of 
sources, residual and pooled. Mobilization of a residual DNAPL, like PCE, can create 
banks that can move ahead of the rich cosolvent flushing phase, or can move downw^ard 
along the cosolvent front, depending on the difference in gravity betv^^een the DNAPL and 
the aqueous flushing phase. Pooled DNAPL can mobilize as described above, but the 
presence of an underlying layer can prevent downward movement if the permeability is 
low enough (high entry pressure), accumulation is small enough, and therefore entry 
pressures into the finer media not exceeded. Under extreme conditions of low entry 
pressure, low interfacial forces, and large pool thickness, entry of the DNAPL in to the 
pores is possible. Eventual breakthrough into lower regions is then probable, depending 
on the layer thickness and amount of DNAPL present. 

Because of the complexities of DNAPL source removal briefly introduced above, 
several questions arise: 1) is there an optimum amount of a cosolvent that can be used to 
maximize solubilization, yet minimize the chance of mobilization; 2) can predictions be 
made as to when mobilization of residual DNAPL will occur; 3) what DNAPL will be left 
behind, or "entrapped," if the entire amount is not removed with the cosolvent used; 4) 
can predictions be made as to whether pooled DNAPL will enter an underlying layer under 
certain flushing conditions; and 5) are there better flushing methods to minimize the 
chance of mobilization of either reskiual or pooled DNAPL? There are a variety of 
chemical, physical, and hydrogeo logic factors that can influence the outcome of these 
questions. Several of these will be discussed in the chapters that follow. 



Selection of DNAPL 

A common solvent used throughout the last few decades is tetrachloroethylene, 
also known as perchloroethylene, or "perc" (PCE, C2CI4, Chemical Abstract Number 
(CAS) 127-18-4). This solvent has been used as an industrial degreaser, but a more 
common use even today is as a dry cleaning solvent. The former use has led to the better- 
known industrial hazardous waste sites. Perchloroethylene has been found in at least 330 
of the 1 1 17 National Priorities List (NPL) hazardous waste sites. However, it is 
increasingly evident that the thousands of dry cleaner establishments in the United States 
had their share of mismanagement of PCE. There are over 600 contaminated dry cleaner 
sites in Florida alone. Due to the sheer number of potential contaminated sites, the 
toxicity of PCE (a drinking water equivalent level (DWEL) of 0.5 mg/L has been 
established by the EPA), and often close proximity of these establishments to residential 
areas, there is growing concern of the impact of long-term subsurface sources of PCE 
contamination. 
Study Objectives 

Based on the background summarized above, the following paragraphs describe 
the objectives of this research. 

Determine solubility relationship of PCE to amount of cosolvent . This objective is 
to determine the solubility relationships (log-linear relationship or other) for a few 
common alcohols used to remediate NAPLs, and justify the quantities used under different 
remediation regimes. Due to the imique nature of water and the various intermolecular 
interactions which occur when a solute is added (or when enough solute is added to 
become a cosolvent), the scientific basis for the solubility relationship can become 



complicated. Once a relationship has been established, an explanation for its features will 
be proposed based on the chemical literature and this study's observations. 

Measure the effects of the type of alcohoKs) and amountCs) on interfacial tension 
(IFT) and develop a relatively simple, vet usefiil relationship between the two . As the 
mutual solubility between two phases increases, the interfacial tension between them 
decreases. A critical feature of cosolvent flushing in the field is the rate of decrease in IFT 
with increasing cosolvent volume fi-actions. This study will measure the relationship 
between the amount of alcohol added and the IFT between the equilibrated phases. 

Propose a relationship between the solubility of PCE into various cosolvent 
mixtures and the resulting equilibrated IFT . It would be beneficial for field applications to 
have an imderstanding of the expected solubility of PCE into the aqueous phase and the 
corresponding IFT that results fi-om this mixture. Mobilization of NAPL is a strong 
fimction of IFT. To have an estimate of the aqueous phase concentration of PCE at 
various IFT values would provide information helpful in determining if mobilization will 
occur under a given flow regime. This study will attempt to establish this relationship to 
possibly be used in fiirther studies and field applications. 

Development of a trapping number relationship using cosolvents . IFT is not the 
only parameter governing mobilization. Relationships have been established (Pennell et al. 
1996b) and applied to surfactant use in porous media. These relationships describe the 
amount of NAPL removed fi-om a given media via mobilization as a fimction of a 
dimensionless "trapping number", which includes contributions from viscous, capillary, 
and buoyancy forces. However, a similar relationship verified with cosolvents has not 
been found in the literature. Sand column experiments were performed similar to Pennell 



et al. (1996) using a cosolvent mixture(s) found from the previous bench top experiments 
to develop and verify the dependence of DNAPL (PCE) saturation on the trapping 
number. 

Determine the relative amounts of mobilization due to purely IFT reduction and to 
possible swelling of DNAPL . During cosolvent flooding both solubilization and 
mobilization of the NAPL can occur. Mobilization occurs due to a variety of 
hydrogeologic and physical parameters. A critical parameter during these remediation 
operations is the IFT. To isolate the effects of reduction of IFT on mobilization of PCE, 
soil column experiments were conducted with the influent cosolvent phase pre-equilibrated 
with PCE. 

Determine the relationship of entrapment of DNAPL to the trapping number and 
the difference between entrapment and mobilization of residual PCE using cosolvents. 
The entrapment of a NAPL in the pore structure after being exposed to reduced interfacial 
tensions is important to evaluate, since a high removal efficiency is desired. The 
entrapment process of PCE in a one-dimensional homogeneous sand column under 
various trapping number environments will be evaluated and compared to the mobilization 
experiments. Additionally, the effect of sand pore size heterogeneity on entrapment were 
observed. „ 

Qualitatively observe the effects of various cosolvent mixtures on the removal of 
PCE pooled above various finer sand layers. Two-dimensional studies were conducted 
varying the amount of cosolvent in the flushing phase and the mode of injection (step input 
versus gradient input). General observations and conclusions were made to improve 
removal of pooled systems using cosolvents. 



g 

Confirm the quantitative prediction of entry of DNAPL into finer pores below 
DNAPL pools, during cosolvent flushing processes. Entry of DNAPLs into finer more 
"impermeable" layers is undesirable during removal of DNAPL plumes. Prediction of 
entry of these DNAPLs into finer layers is straightforward via basic force balance 
calculations. However, under cosolvent flooding conditions, parameters used to calculate 
entry pressures (density contrast and interfacial tension between the two fluids) are 
changing during flooding processes, especially for strongly partitioning alcohols like t- 
butyl alcohol. It is possible that the entry predictions can be made assuming equilibrium 
density and interfacial tension values. 

Dissertation Organization , 

Each of the following chapters (Chapters 2-5) is written to essentially be a stand- 
alone document. Thus, a similar format of Introduction, Methods and Materials, Results 
and Discussion, and Conclusions is used throughout. Chapter 2 includes the results of the 
bench top solubility and interfacial tension studies (first through third objectives). Chapter 
3 discusses the results of the mobilization studies of residual perchloroethylene (fourth and 
fifth objectives). Chapter 4 fiirther expands on the process of mobilization and its 
comparison to the entrapment of DNAPL after exposure to reduced interfacial tension 
conditions (sixth objective). Finally, Chapter 5 outlines the results of the two-dimensional 
box studies to evaluate flooding processes on pooled DNAPL system and predict their 
entry into a finer medium below (final two objectives). Chapter 6 summarizes the major 
conclusions of the entire dissertation and identifies areas of fiiture research. 



CHAPTER 2 
INVESTIGATIONS OF THE RELATIONSHIP OF COSOLVENT FRACTION TO 
PERCHLOROETHYLENE (PCE) SOLUBILITY AND EQUILIBRIUM 

INTERFACIAL TENSION 



Introduction 

Optimization of remediation technologies is a prime concern to engineers, 
scientists and environmental regulators. This optimization involves engineering, scientific, 
economic, environmental impact and health risk considerations. One recent technology 
for Non- Aqueous Phase Liquid (NAPL) or Dense Non- Aqueous Phase Liquid (DNAPL) 
removal fi-om subsurface environments is the use of cosolvents to increase the 
solubilization of contaminants and to flush the mixtures (and NAPL if mobilization occurs) 
into recovery wells. Cosolvents are commonly binary alcohol-water mixtures and less 
commonly ternary alcohol A-alcohol B-water mixtures. The exact "recipes" for these 
mixtures are rather loosely chosen. Relationships between the solubilities of the 
contaminants of interest and the resulting interfacial tensions (IFTs) for different amounts 
of cosolvent would be beneficial to optimization of these technologies. Hereafter, the 
term IFT, and IFT measurements presented in the results section, are defined (or 
measured) at the interface between the cosolvent mixture and the DNAPL phase. One 
common DNAPL is perchloroethylene (PCE), a historically common industrial degreaser 
and dry cleaning solvent. An objective of this investigation was to determine the effects of 



10 

various cosolvents and cosolvent mixtures on the solubility of PCE and the resulting 
equilibrium interfacial tension between the two phases (organic and aqueous). From these 
results, it is desired to develop a simple predictive relationship between these factors. An 
accurate understanding of the IFTs of these mixtures also allows better prediction of the 
mobilization of a separate PCE phase. This situation is a concern due to the density of 
PCE (approximately 1 .62 g/ml) and its possible downward movement (mobilization), out 
of the control of the remediation scheme. 
Comparison of the Molecular Structures of Water and Low Molecular Weight Alcohols 

Cosolvents typically used for cosolvent flooding operations are monohydric 
(contain only one alcohol group, e.g., ethanol or isopropyl alcohol). When these 
monohydric alcohols are present in binary alcohol/water mixtures, deviations from ideal 
solution behavior are seen, especially at lower volume fractions (Franks and Ives 1966). 
These deviations can be attributed in a general way to the bifimctional nature of these 
types of solute molecules. It is a push-and-pull effect where the small hydrophobic 
portion of the molecule resists the pull exerted by the hydrophilic hydroxyl group. This 
hydroxyl group, either as a proton donor or acceptor, can hydrogen bond with the solvent 
(water) molecules. Although hydrogen bonding plays a role in the behavior of these 
systems, it cannot account for all observed phenomena. Other structural differences need 
consideration. Deviations from ideal behavior is noticeably lessened if a second hydroxyl 
group is added to the molecule (e.g., glycols) which shifts the balance of forces in favor of 
a more "aqueous behavior" (Franks and Ives 1966). 

The structure of water is tetrahedral in shape, with the polar 0-H bonds being 
approximately sp^ hybridized. Thus, each oxygen atom can form approximately four 



•:- ■• -iv:.: 11 

tetrahedrally-disposed hydrogen bonds (Frank and Wen 1957). Formation of these 
hydrogen bonds is energetically favorable, until it suffers collective destruction by high 
energy fluctuation. Thus, a three-dimensional cluster of water molecules is formed, which 
lifetime is on the order of 10"" seconds (Franks and Ives 1966). Although this lifetime is 
short, it is still two or three orders of magnitude greater than the period of molecular 
vibration. Liquid water is considered to be a mixture of these clusters (which can be 
open) and a dense fluid composed of non-hydrogen bonded water molecules (Franks and 
Ives 1966). This order-disorder balance in water is sensitive and is highly significant to its 
properties. This is particularly the case in the reaction of water with hydrophobic parts of 
bifunctional molecules, like alcohols. 

For aliphatic alcohols, like ethanol, the C-H bonds are sp^ hybridized, with the O- 
H again being close to the same hybridization. Similar to water, it can form hydrogen 
bonds between alcohol molecules, but generally no more than two hydrogen bonds can 
form (each oxygen acting as a proton donor and as an electron acceptor). Linear 
polymers of 5-7 molecules (or less for stericaUy hindered alcohols) are formed, with 
lifetimes on the order of 10"" to 10"' seconds (Magat 1959). It is clear that hydrogen 
bonding has a significant effect on the properties of alcohols, but not in the same way as 
water, in which increasing hydrogen bonding leads to more cavity formation, or an 
"openness" of structure. Franks and Ives (1966) consider an 80% mole fi-action 
ethanol/water mixture (similar to the concentrations used in cosolvent flooding processes) 
and note that the number of "moles" of hydroxyl group per mole of liquid phase for pure 
water, the mixture, and pure ethanol (EtOH) are 2, 1.2, and 1, respectively. Even more 






12 



significant are the concentrations of protons available for hydrogen bonding in these 
liquids - 1 1 1, 24, and 17 moles/l (Franks and Ives 1966), as shown in Figure 2-1. 



120 



I 100 

E 

I 80 
& 

>> 

X 

o> 

c 

•a 
c 

i 40 

X 

o 

o 20 



60 



0.1 0.2 0.3 0.4 0.5 0.6 0.7 

Mole Fraction Ethanol 



0.8 



0.9 



Figure 2-1. Graph of data from Franks and Ives (1966), relating concentration of 
hydrogen bonds to volume fi-action of ethanol. 



It is therefore much more difficult for a hydrophobic solute like PCE to enter into 
this network at lower mole fi-actions of EtOH. This may account for negative deviations 
in log-linear behavior of the solubility of PCE at lower EtOH fi-actions (Morris et al. 
1988). Thus, it takes higher concentrations of EtOH molecules to form their own 
network of hydrogen bonded polymers, that consist of a larger area of hydrophobic 
properties, to which PCE can intermolecularly bond. 

PCE (C2CI4) is a symmetric molecule, therefore non-polar. However, the four C- 
Cl bonds are locally very polar, and can thus lead to dipole-dipole intermolecular bonding 
with other molecules. This can explain why PCE has a solubility in water (150 mg/1; (Lide 



13 

1996)) higher than a non-polar molecule with lower localized polarity, like hexane (1 1 

mg/1; (Lide 1996)). However, water, having all polar characteristics within the molecule, 

is not a similar environment for a non-polar solute to enter. The hydrogen-bonding 

network decreases this possibility further. Hence, the relatively low solubility. PCE is 

completely miscible with EtOH, due to decreased hydrogen bonding (as compared to 

water) and the presence of a hydrophobic portion of the EtOH molecule, which leads to 

strong dispersion forces between the two. Additionally, dipole-dipole forces are present 

between these two compounds. 

Solubility of Hydrocarbons into Water/alcohol Mixtures and its Relationship to Amount of 
Cosolvent 

An ideal solution can be defined as one that does not deviate fi^om Raoult's Law 
(Atkins 1994): 

Pa=XaPl, (2-1) 

where pa [ML'T^] is the vapor pressure of a in the liquid (binary for our purposes), Xa is 
the mole fi-action of a in the liquid, andpa is the vapor pressure of the pure liquid a. 
Using Raoult's Law, for an ideal solution: 

M,=fil+RT]nx,, (2-2) 

where jUa [ML^T^moles"'] is the chemical potential of a in the liquid, /^ is the chemical 
potential of the pure liquid a at standard state, R [ML^T'^moles"' degrees' '] is the universal 
gas constant, and T [K]is the absolute temperature. The chemical potential of substance a 
expresses how the free energy of the system changes as a is added (Atkins 1994). As can 
be seen from Equation ( 2-2), how this potential changes depends on the composition of 



the system (xa). The chemical potential, hence the free energy of the system, is held to this 
relationship for an ideal solution. Physical properties like solubilization and IFT that 
depend upon the free energy of the system are therefore strongly linked to this 
relationship, often exhibiting log-linear behavior with composition, especially in more 
dilute solutions (Chen and Delfino 1997; Morris et al. 1988). Adding alcohols to water to 
enhance the solubility of contaminants during remediation processes is one example of 
where this miscibility relationship is beneficial. However, volume fractions of cosolvents 
used are generally high (70-90%) (Annable et al. 1996) and deviations from ideal solution 
behavior are often observed. 

Solubility estimation methods most commonly used ("mixed solvent solubility 
estimation methods") assume that the solvent molecules are randomly mixed. Therefore, 
deviations from these models (in a positive sense) indicate that in organic/solvent water 
systems the solvent molecules are not randomly mixed. Increased deviations from random 
mixing with water occur as cosolvent molecular size increases and hydrogen bonding 
capability decreases (Dickhut et al. 1991). This is due to cosolvent self-interaction 
increasing, providing a more desirable environment for hydrophobic solutes in aqueous 
solution, and decreased hydrogen bond "networking" allowing the solute to move more 
freely and find more desirable zones. 

Non-ideal binary monohydric alcohol and water mixtures have been studied for 
quite some time. Use of alcohols as industrial solvents has also prompted more detailed 
studies. Solubility relationships of various hydrocarbons in these mixtures to the amount of 
alcohol present have been determined (Dickhut et al. 1989; Groves 1988; Pinal et al. 
1990). The most prevalent relationship used is log-linear. 



. - - 15 

It has been shown that a 70% ethanol/18% water/12% n-pentanol mixture can be 
used to solubilize various hydrocarbons from contaminated media (Annable et al. 1996; 
Rao et al. 1997). Binary methanol, ethanol and isopropyl alcohol/water mixtures have 
also been used (Augustijn et al. 1994; Brandes 1992; Imhoflf et al, 1995). Typically, high 
volume fractions (>80% v/v) of alcohol are used. Specific studies on the superiority of 
these mixtures in cosolvent flushing applications could not be found. However, as the 
fraction of cosolvent increases, the aqueous solubility of NAPL constituents increases 
(Brandes and Farley 1993). However, monohydric alcohols, like ethanol, and water 
binary mixtures have been shown to behave non-ideally. Simple relationships like the log- 
linear model (Li et al. 1996; Yalkowsky et al. 1976) may not be applicable over the large 
volume fraction range possible for the use of cosolvents. _ 

Log-linear relationship . i" . 

The log-linear model is used quite frequently when describing cosolvent systems. 
Yalkowsky (Banerjee and Yalkowsky 1988) and others have shown that in solutions of 
appreciable (>10% v/v) cosolvent, the molar solubility of a non-polar solute can be 
approximated by: > , 

log Sm =/clog Sc + (l-/c)log Sw, ( 2-3 ) 

where Sm, Sc, and Sw are the solubilities of the non-polar solute in the mixture, pure 
cosolvent, and pure water, respectively, and fc is the cosolvent volume fraction. This 
equation neglects solute-solvent interactions and is based upon the accepted linear 
relationship between the free energy of solution and the solute surface area (Valvani et al. 
1976). The model is exactly obeyed only as the solvent components become identical (Li 



> 16 

and Andren 1995). The log-linear model assumes the water and cosolvent behave as two 
distinct entities and neglects the interaction between them. This model fails when 
interactions between solvent components are strong and differ from interactions among 
molecules of individual pure components and when the solute strongly prefers one solvent 
component over the other (Li and Andren 1995). Over the total volume fraction 
spectrum, deviations obviously occur mostly at both extremes, where one of the solvents 
is present at very low concentrations and cannot avoid interaction with the other solvent. 
At very low cosolvent volume fractions, the solute solubilized will only be influenced by 
one cosolvent molecule at a time. Any solubility enhancement will therefore be 
proportional to the number of cosolvent molecules present. This cosolvent will be 
hydrated in solution, and consequently, it will disrupt the water network structure 
(Grunwald 1986). In these situations, one would expect the log-linear relationship that 
applies at higher cosolvent fractions to become linear, due to a change in the mechanism 
of solubiliaation (Banerjee and Yalkowsky 1988). This change usually occurs in the range 
of 0.1 <fc < 0.2. The cosolvent/water mixture behaves as a completely random 
arrangement of molecules with no tendency to segregate (i.e., an ideal mixture) (Dickhut 
et al. 1991). At these low volume fractions, cosolvents are more like coso lutes in 
behavior and do not influence the solution in an appreciable way. 

For a remediation process, cosolvent volume fractions are typically on the order of 
80%; so the minor cosolvent is water. Any operation therefore in the 80 to 100% range 
could possibly be in the linear portion of the solubility relationship discussed above. The 
primary advantage of the log linear method is its simplicity, which makes it a convenient 



17 

tool for estimating solubilities of hydrophobic chemicals in a variety of aqueous mixtures 

(LiandAndrenl995). 

Cosolvency power 

The relative solubilization enhancement is usuaUy presented as the "cosolvent 
power", o (Banerjee and Yalkowsky 1988; Yalkowsky et al. 1976). This cosolvency 
power is defined as the logarithm of the ratio of the solute solubilities in pure cosolvent to 
pure water, or 

o - log 5c - log 5m/. ■ (2-4) 

In some instances the solute may be completely miscible in the pure cosolvent (i.e., PCE in 
ethanol) where the use of the "end-to-half- slope" (ao.s) is necessary (Li et al. 1996). This 
is defined as 

■ a,, = (log5,,-log5J/0.5, (2-5) 

where Sg ^ is the solubility aty^ = 0.5. In combination with Equation ( 2-3), this results in 
the expression, 

logS„=cTo,J,+logS^. (2-6) 

Other methods for solubility estimation 

To account for deviations fi"om ideal or regular solution theory, other methods 
have been developed in previous research. These include the Extended Hildebrand (EH) 
method, the Excess Free Energy (EFE) method, and the Universal Functional Group 
Activity CoeflRcient (UNIFAC) method. 



»* - -* 



18 

Extended Hildebrand (EH) method . Hildebrand and Scott (1950) and Scatchard 
(1931) introduced regular solution theory to describe solutions that maintain ideal entropy 
of mixing, but involve heat change during mixing. This can occur only if the random 
distribution of molecules is maintained even in the presence of specific solute-solvent 
interactions (Barton 1975). However, solutions of organic compounds in polar solvents 
are not regular since significant solvation can occur (Li and Andren 1995). To attempt to 
account for deviations fi-om ideal behavior, Martin et al. (1979; 1982) assumed the binary 
cosolvent and water mixture is itself ideal, but the ternary (or higher) solution behavior 
may deviate fi-om the ideal due to solute(s)-solvent interactions. This method is 
represented by the following expression for the mole fi-action solubility: 

^x,^^=^(T-TJ-^(SL+S^-2W), (2-7) 

where 

(^.^=^A+ ^2^2 +■■■)• (2-8) 

and Xsj is the mole fiaction solubility of solute s in solvent/ (/-1 ,2,3..); As^ [ML^T^ moles" 
'degrees K"'] is the molar entropy of fiision; R is the ideal gas constant; Tand !„ are the 
absolute system and melting temperature; qs [L^moles'] is the molar volume of the solute; 
zj is the solute free volume fractions of the solvents in the mixture; 5$, and 5j are the 
Hildebrand solubility parameters for the solute and each solvent, respectively; and W [ML" 
T" ] is the interaction energy in those systems with strong solute-solvent interactions. 
This estimation technique requires the determination of the solute specific interaction 



19 

energy, W. The EH method is most usefiil in situations in which solubility determinations 
for a specific solute are desired, as in this study (Dickhut et al. 1991). 

Excess fi-ee energy (EFE) method . The EFE method (Williams and Amidon 
1984b) accounts for some deviations from the log-linear prediction. Non- ideal solution 
behavior is attributed to excess free energy from n-body interactions in the given system. 
By ignoring four-body and higher order interactions, the model is reduced to a three-sufiBx 
equation for the mole fraction solubility of a solute in a mixed solvent. For a ternary 
system (solvent, cosolvent and solute) this model is given as: 

lnx,„„ =z, ]nx,^ +z^ Inx,^ -A^_^z^2^{2z^ -1)(^, I q^) + A^_p.z\z^{_q^ /q^)+Qz^z^, ( 2-9) 

where ^/.^and A^j are the binary solvent-cosolvent interaction constants; q^ is the molar 
volume of the species i; and C, is the ternary interaction constant. This method requires 
vapor/liquid equilibrixim data (at the system temperature) to derive the solvent-cosolvent 
interaction constants. However, ^,2 and ^j., can also be calculated using UNIFAC data. 
The ternary interaction constant, C^, requires, in practice, solute solubility measurements 
over a range of cosolvent fractions to determine this parameter (Williams and Amidon 
1984a). This method is less acceptable because it relies on the experimental solubility data 
to determine the parameters needed for mixed solvent solubility estimations (Dickhut et al. 
1991). Furthermore, specifically for the systems studied herein, the solute (PCE) is 
completely miscible in one of the solutes (ethanol or isopropyl alcohol). Therefore, the 
mole fraction solubility of PCE in ethanol is undefined. This eliminates this model as a 
tool to predict PCE solubility in these systems. 



20 

UNIFAC method . The UNIFAC method uses the sizes and shapes of molecules in 
the solvent mixture and the interactions between the fiinctional groups they contain to 
account for the non-ideal solution behavior (Fredenslund et al. 1977). Its fundamental 
assumption is that the chemical behavior of a fluid is due to the sum of contributions made 
by the molecules' functional groups. This method calculates the activity coefficients (y-) 

based on the fimctional groups of a molecule of species / and their interactions with other 
groups in the system. It is given as: 



ln^,.m« 



~RT 



(T-TJ-]ny,^„„, (2-10) 



where .'...' 1'^ .'^ ' -' '"."'" . ,-,. . ':■% 

■X' ■ " 

^rs.n,.=^r''+^r\ (2-11 ) 

and 7j^^ is the UNIFAC activity coeflScient of the solute in the solvent mixture, f is the 
combinatorial fraction and / is the residual fraction. The combinatorial fraction reflects 
the size and shape of the molecules, and the residual fraction depends on the functional 
group interactions. Parameters for each functional group, such as volume and area 
parameters (normalized van der Waals volume and surface areas) and parameters of 
interaction with other functional groups (obtained from phase equilibrium experimental 
data) are put into a series of equations to calculate /and f. The UNIFAC method is 
limited by the experimental data used to determine its parameters in Equation ( 2-11), 
some of which have been updated since the inception of this method (Hansen et al. 1991). 



21 

Choice of solubility estimation method 

Whatever the method finally chosen to best represent PCE/cosolvent/water 
behavior, some general conclusions have been made in the literature. As the cosolvent 
molecular size decreases, the hydrogen bonding capability increases. This leads to 
significant non-ideal behavior. This indicates that in these types of systems (especially 
ethanol and isopropyl alcohol) the solvent molecules are not randomly mixed. Self- 
interaction among organic solvent molecules increases and is hydrophobic. This creates a 
more desirable environment for hydrophobic solutes, increasing solubility higher than 
expected in ideal solvent mixtures (Dickhut et al. 1991). It is generally accepted that no 
single model is able to accurately predict hydrophobic species solubility in any system, 
especially over a wide range of environments, such as increasing cosolvent fi-actions. Until 
one such model is developed, use of the best resulting fit to the experimental data 
produced will have to be sufficient. 

If solubilization relationships are characterized, it may be possible to use a lower 
fi-action of alcohol and have similar NAPL solubilities, by selecting a better cosolvent 
mixture. Given economic, hydraulic, and environmental factors, even a cosolvent that 
results in NAPL solubility slightly less than a possible competitor could potentially be a 
viable candidate in the field. This is especially important if mobilization of NAPL is 
strongly undesirable. Interfacial tension is the key parameter that can determine whether 
mobilization will occur or not, and is discussed below. 
Interfacial Tension of Ternary Alcohol/water/PMOS Mixtures 

Solubilities of hydrophobic organic compounds (or Partially Miscible Organic 
Substances, PMOS) are strongly dependent upon the nature of the interfaces between the 



22 

two phases in organic/aqueous phase systems. The cohesional and adhesional forces of 
the molecules of a liquid- liquid system are the factors that determine the extent to which a 
given solute is soluble in a given solvent. It is these factors which also determine the 
magnitude of the interfacial tension (IFT) (Donahue and Bartell 1958). IFT is a critical 
parameter necessary to characterize this interface and to characterize non-homogeneous 
liquid systems (Glinski et al. 1994). It is often necessary to know the IFT to predict the 
fete of organic liquids. IFT is defined as the change in Gibbs fi-ee energy per unit change 
in interfacial area. 

— =r • (2-12) 

Accurate estimation of this parameter is critical to predict behavior of liquid phases 
during field remediation operations. To predict IFT, semi-empirical formulae are used. 
These include Antonov's Rule (Antonov 1907), and the methods of Girifelco and Good 
(1957), Donahue and BarteU (1958), and Fu (1986). 
Relation to amount of cosolvent 

Consider only two dissimilar liquids. The IFT between the two liquids results fi-om 
an imbalance of forces acting on molecules at the interface. The IFT value is a function of 
the interaction between not only the molecules of the two different liquids, but also the 
molecules of the individual liquids themselves. The magnitude of the IFT reflects the 
relative difference between intermolecular forces within the bulk liquid and the 
intermolecular forces between the liquids (Demond and Lindner 1993). This can be 
extended to more than binary systems, with increasing intermolecular interactions to 
consider. 



23 

As a cosolvent is added in increasing proportion to an aqueous mixture the 
interfacial tension between it and a separate organic phase decreases. This decrease is due 
to the increasing similarity between the two phases. The cohesional forces within the two 
phases are high when the phases are dissimilar, resulting in excess free surface energy, or 
interfacial tension. As cosolvent is added to the aqueous phase, these cohesional forces 
decrease, decreasing the IFT. At the same time, the increasing similarity between the two 
phases increases the mutual solubility of the solutes within each phase. The historical 
literature has recognized this relationship between solubility, IFT, and the amount of 
cosolvent added. The major estimation techniques are described below. 
Different interfacial tension prediction methods 

Method of Fu et al . The only method at the present time to estimate IFT for 
ternary (or quaternary) systems is the one developed by Fu et al. (1986). It is derived 
from the thermodynamic equation developed by Shain and Prausnitz (1964) 



RT, 



V;< ^ 



(2-13) 



where i? is the universal gas constant, T is the absolute temperature, /■ and / . are the 
activity coefficients of component / in the interfacial and bulk phases, respectively, x- and 
Xi are the mole fractions of component / in the interfacial and bulk phases, respectively, 
and A." is the partial molal cross-sectional area of component / in the interfacial phase 
[L mole' ]. This can be applied to any mixture containing any number of components, as 
long as values for the thermodynamic parameters are known (or estimated). This method 



24 

makes a couple of simplifying assumptions that lead to the final expression for calculating 
the interfacial tension of a ternary mixture. 

where Kisan empirical constant (relating the number of molecules in the interfacial phase 
to the ratio of the molecular cross-sectional area to its surface area); 

9 

X=^ -ln(x,+X2+X3^); A^ is the van der Waals surface area of a standard segment (2.5 x 10 
cm^/mol, (Abrams and Prausnitz 1975)); xj is the mole fi-action of the rth component in the 
phase where that component is a solute, x^^ is the mole fi-action for the third component in 
the phase where it is richer; and gi - A^/A^^, where ^^j is the van der Waals surface area 
for component / (qj is the pure component area parameter defined by the UNIQUAC 
model) (Abrams and Prausnitz 1975). 

The value of A" is suggested by Fu et al. (1986) to be 0.9414, based on a linear 
regression of 54 binary systems. The average relative deviation fi-om the measured IFT 
was 23%. However, if only data with IFT greater than 10 dyne/cm are considered, this 
deviation decreases to 6.3% (Fu et al. 1986). With the value of K taken to be 0.9414, Fu 
et al. tested 23 ternary systems and the average relative deviations were 17.9% for 
mixtures with IFT > 5 dyne/cm, and 1 1 .5% if only those data points with and IFT > 10 
dyne/cm are considered. However, this may be a significant, especially if trying to predict 
a value during co solvent flooding operations when IFTs can decrease well below 10 
dyne/cm. Additionally, for some systems either x^^ or Xj (mole fi-action of the third 



25 

component in the richer or poorer phase) can be chosen to obtain a better correlation. 
The exact cause of this phenomenon is not clear. 

Donahue and Bartell . Donahue and Bartell (1958) relied on the feet that 
miscibility and IFT reflect the same intermolecular forces. They discovered there was a 
linear relationship between the IFT of liquid pairs and the log of the sum of mutual 
solubilities in binary systems. 

ro,v=^-bHSo(m + ^mo)) (2-15) 

where a and b are en:^)irical constants (regressed intercept and slope, respectively), S^^ is 
the mole fraction solubility of the organic in water, and S^^^^ is the mole fraction solubility 
of the water in the organic. For higher order systems it is obvious that this method cannot 
be applied directly. However, an additional relationship of the IFT being a fimction of the 
mutual solubility of the cosolvent alcohol and the NAPL is still possible. 

Others . The oldest method still in use to estimate interfecial tension is Antonov's 
rule (Antonov 1907). It is stated by the relationship 

yow ~ Yw^o) ~ Voifv) ' ( 2-16 ) 

where /^^ is the estimated IFT between the organic liquid and water, /^(O) ^ ^^ surface 
tension of water saturated with the organic, and /q^^^^ is the surface tension of the organic 

saturated with water. As this is for a binary system only, its applicability to cosolvent 
systems is unfounded. Furthermore, use of this method for PCE systems has been shown 
to be inaccurate (Donahue and Bartell 1958). 






26 

Girifalco and Good's method is derived on the basis of the work necessary to 
separate the liquids at their interface. They assumed that the potential energy function for 
the interaction across the interface was described by the geometric mean of the IFTs 
(Demond and Lindner 1993). This method states that the IFT of a binary system is: 

;■, row = ro + rw^2^irorJ'\ (2-i7) 

where O is the interaction parameter describing the similarity of intermolecular force 
between the two liquids, and Xo ^nd y^, are the interfacial tensions between the oil phase 

and air, and water and air, respectively. The value of O ranges from 0.5 to 1 . 1 5 for 
organic liquid water systems, with lower values associated with dissimilar liquids and 
higher values associated with similar systems (Demond and Lindner 1993). Again, this is 
a method applicable to only binary systems. 
Choice of IFT estimation method 

Antonov's rule and Girifalco and Good's method are either too simplistic (lack a 
theoretical basis) or do not perform well, respectively (Demond and Lindner 1993). 
Girifalco and Good's method has a theoretical base, but is applicable only to binary 
systems. The most accurate methods appear to be those of Fu et al. (1986) and Donahue 
and Bartell (1958) (Demond and Lindner 1993). Donahue and Bartell's method performs 
better if measured mutual solubility data are available. Fu's method is preferred in cases 
where the mutual solubility data must be estimated. However, both of these methods have 
lower accuracy for systems where the IFT is less than 10 dyne/cm (Demond and Lindner 
1993). This region of IFTs is where cosolvent flushing schemes will transition from 
maximizing solubility or mobilization of the contaminant. Furthermore, Fu's method is the 



27 

only one to directly apply to ternary systems. This method or a more direct empirical 

correlation is favored in this study. 

Relation of IFT to Solubility of Organic Solute 

As mentioned previously, there are various methods that can estimate the 
relationship between the solubility of a PMOS into a cosolvent/water phase and the 
amount of cosolvent added. Correspondingly, a relationship exists (Fu's method) between 
the IFT and the mole fraction solubility of a third solute (cosolvent). Therefore, with 
proper connection between the two relationships, one should be able to determine the 
dependence of the solubility of a PMOS with the equilibrated interfacial tension of the 
system. In a subsurface system, this type of direct relationship between these important 
parameters will allow quick conparison of enhanced solubility with the predicted IFT. If 
the remediation technique is designed to solubilize and not mobilize a DNAPL plume, 
there could be situations where driving to increase the solubilization of the DNAPL would 
result in significant lowering of IFT, and hydraulically move the system into mobilization 
regimes. If mobilization is a concern, a lowering of the cosolvent fraction a given amount 
may safely increase the IFT and only impact solubility by a small factor. This could lead to 
only a few additional pore volumes of the flushing fluid, or fractions thereof, used to 
obtain similar mass recovery of the DNAPL, while ensuring less risk of mobilization. 

Materials and Methods 

All chemicals were obtained from Fischer Scientific and were chromatography 
grade, with the exception of ethanol (EtOH). 99.9% EtOH was obtained from Ultra- 
Chem Corporation. Various nuxtures of cosolvent and water were made in 40 ml EPA 



% 28 

vials with Teflon-lined screw caps. The resulting mixtures were defined by the volume 
fi-action of cosolvent (/^) in the aqueous phase, which were calculated fi-om the amounts of 
water and alcohols that were measured separately and then combined in preparing the 
solvent mixtures (Li and Andren 1994). Ten milliliters (ml) of PCE and 10 ml of the 
cosolvent mixture was added to each vial, so that the initial ratio of aqueous to NAPL 
phases was 1:1. The vial was then placed in a mechanical rotator and rotated at room 
temperature (25±0.5 °C) for 48 hours, removed and allowed to settle for 24 hours. A 1 to 
2 ml sample of the aqueous phase was then taken and placed in GC autosampler vials and 
crimp sealed with Teflon lined caps. The remaining fluid in each vial was then carefully 
poured into a straight-walled glass crystallization dish (50 mm diam. x 35 mm depth), 
which had been previously cleaned for IFT measurements using the procedure of Wilson 
et al. (Wilson et al. 1990). After a few minutes of settling, the IFT was measured with a 
Du Nuoy ring tensiometer, Fisher Scientific Model 21 Tensiomat. The solubility of PCE 
was determined by injection of 1 nl of the equilibrated aqueous phase into a Perkin-Elmer 
Autosystems GC. The GC column used for this study was a 30 meter x 0.530 mm, 3 (im 
fixed phase, DB-624 column, manufactured by J&W Scientific. Column conditions were 
set at 35°C for 2.5 minutes, then ranq)ed up 6 degrees per minute to 95°C. The carrier 
head pressure was set at 4 psig. A flame-ionization detector (FID) was used in the 
analysis for PCE. Although the detection for PCE is much improved using an electron 
capture detector (ECD), the concentration range of interest was jfrom 150 ppm to 20,000 
ppm. With these higher concentrations and the wide range of possible results, the linearly 
responding FID was chosen over the ECD. Additionally, simultaneous analysis for other 
components (non-halogenated) was also possible. 



29 

Results and Discussion 

Initial data collection has been conducted for various cosolvent/water/PCE 
mixtures. Results of solubility measurements support the non-ideality of alcohol/water 
mixtures. The results show a sigmoidal type relationship of solubility to the original 
volume fraction of cosolvent (Figure 2-2). 
Log-linear Solubility Estimation 

Ethanol 

Using the relationships described in Yalkowsky et al. (1976), a cosolvency power 
for EtOH was determined to be oo 5 = 3.73. This "end-to-half-slope" cosolvency power is 
applicable when the solute (PCE) is completely miscible with the cosolvent (ethanol), and 
is calculated by Equation ( 2-5). The cosolvency power is then used as the linear slope in 
Equation ( 2-6). The resulting log-linear prediction is shown in Figure 2-2 for reference. 
Solubility of PCE in low volume fraction EtOH mixtures {fc < 0.35) is below the log-linear 
predictions. This is possibly due to an insuflScient quantity of EtOH to frilly influence the 
mixture as a cosolvent and the strong hydrogen bonding network of water still present, as 
discussed above. Furthermore, at these low EtOH fractions, the PCE solute molecule may 
only be influenced by one cosolvent molecule at a time (Li and Andren 1995). 
Correspondingly, at higher volxmie fractions (fc > 0.5) PCE solubilities are slightly above 
those predicted for the log-linear relationship. Ethanol present at high fractions 
overwhehns the water molecules and the cavity network structure so that hydrogen 
bonding is no longer a large factor (Franks and Ives 1966). Increases at higher /^ could be 
due to the self-alignment of the ethyl groups of ethanol molecules, presenting a more 



30 

fevorable organic "zone" for PCE partitioning, and breakiog the three-dimensional 
hydrogen bond network of water completely. PCE favors solubilization in alcohols much 
more than water, and this strong preference leads to failure of the log- linear method (Li 
and Andren 1995). 



loocnoo 



100000 



10000 



CO 



s 



1000 



100 

















11 ^y^ 










■' > 


ir '^ 






Log-linec 


r predictio 
\ 


1 ^^ 


1 

/ 


1 

\ 












,1^— * 




Y 


1 















10 20304050607080 

Volume % of Cosoivent 



90 100 



♦ ethanol « ethanol(2)— ■ isopropanol 



Figure 2-2. Solubility of PCE as a fimction of cosoivent volume fractions (initial phase 
voliimetric phase ratio 1:1). 



Isopropanol (IP A) 

IPA shows increased solubility of PCE as compared to EtOH. The decreased 
polarity of the IPA molecule (relative to EtOH), increase in hydrophobicity, and the 
decrease in hydrogen bonding due to steric hindrances allows for increased amounts of 
PCE to solubilize into the cosoivent mixture. The resulting cosolvency power is 



31 

approximately 4.13, an average based on the solubilities of PCE at/c = 0.4 and/c= 0.6. 
The solubility of PCE at/EtoH = 0.8 is approximately the same as that at an IPA fraction fc 
= 0.7. This example of reduction in cosolvent use can be economically and politically 
beneficial in some field scenarios. Typical costs for these solvents are $0.40/lb and 
$0.35/lb for ethanol and isopropyl alcohol, respectively (Chemical Marketing Reporter, 
1995). Therefore, cost savings can be obvious. 
UNIFAC Method 

Results of the solubility estimations are plotted with those predicted by the 
UNIFAC method and are presented in 

Figure 2-3. The UNIFAC method appears to be adequate for describing cosolvent 
effect on the solubility of PCE. Leirge deviations (under predictions) occur at lower 
volume fractions for isopropanol, most likely due to the inability of UNIFAC to properly 
account for solute-cosolvent/solvent interactions at these lower cosolvent fractions. 
UNIFAC estimations improve quickly at^^ = 0.2 and differences between actual and 
predicted values remain the same as the volume fraction approaches those most likely used 
in remediation scenarios. Thus, the method may be adequate for quick approximations in 
these systems. 
Extended Hildebrand Method 

The solubility parameters for the various components are given below in Table 2-1 . 
It has been reported (Martin et al. 1982) that when the range of solubility parameters of 
the solvent pair approaches the solubility parameter of the solute, the curve may bow 
sufficiently that a log-linear expression of solubility on volume fraction of cosolvent no 



32 



i.E-rt»-| 
















• 














• 


♦ 





U 

a. 

■s 

c 












• 














ry 
















« 


8 

♦ 
















♦ 












IE-OS - 


♦ 


• 

o 






























1 F-06 - 


o 














1 1 



0.1 



0.2 0.3 04 0.5 0.6 0.7 

Initial volume fraction of cosolvent 

I ♦ Ethanol Data • IPA Data O UNIFAC Ethanol o UNIFAC IPA I 



0.8 



0.9 



Figure 2-3. Comparison of Measured Solubility Data and those predicted by the UNIFAC 
Method. 



Table 2-1. Solubility parameters for study components (Barton 1975) 



Component 


Solubility Parameter (cal/cm^)"^ 


Water 


23.4 


PCE 


9.3 


ethanol 


12.7 


isopropyl alcohol 


11.5 



longer fits the data satisfectorily. A quadratic or higher polynomial must therefore be used 
as required by methods such as the extended Hildebrand method. Thus, the log- linear 
approach, even though it is often usefiil, should be used with caution over a wide range of 



33 

cosolvent volume fractions (Martin et al. 1982). To apply the extended Hildebrand 
approach takes a little more effort, but it usually reproduces the solubility in mixed solvent 
systems better over an entire range of solvent compositions. 

The log-linear method would seem to apply to such a system as ethanol/water/ 
PCE, since the solubility parameter of the solute is 3 to 4 imits below that of the organic 
solvent (ethanol) (Martin et al. 1982). However, as seen in Figure 2-4 below, the 
Extended Hildebrand theory predicts the solubility of IPA better than either UNIFAC or 
the log-linear method. This is due to the inclusion of solute-solvent interaction, which is 
important when the solute (PCE) is more miscible in one of the solvents (isopropanol) 
than the other (water). The Hildebrand method is incorrect throughout the entire 
cosolvent regime, but improves as the solvent-solvent interaction assumption becomes 
more valid as isopropanol becomes the primary solvent, similar to most cosolvent 
remediation scenarios. 

The main advantage of the extended Hildebrand approach is that it handles solutes 
in polar and non-polar systems, whether the solute's solubility parameter is greater than, 
less than, or lies between the solubility parameters for the solvent pair (Martin et al. 1982). 
Although the extended Hildebrand is widely applicable, some corrections are needed in 
various situations. These include a correction factor for the entropy of mixing to account 
for the differences in molecular size (Flory-Huggins correction term) and a term for the 
additional entropy effects associated with hydrogen bonding substances (Amidon et al. 
1974). For water/alcohol solvent systems, this is especially true. Transfers of small 
hydrocarbons from nonpolar liquids to water are accompanied by large negative entropies 



34 



1.E+00 



1.E-01 



i.E-a2 



; 

c 



1.E-03 



1.E-04 



1.E-05i^ 



1.E-06 



1.E-07 



■■■*■■ A 
A 

.: — ^ _ _: — ^ 

* 

2 

A 

a 

li 

♦ 



0.1 0.2 0.3 0.4 0.5 0.6 

Inital volume fraction of cosoivent 

I A IPA Data ♦ Hildebrand a Ext. Hildebrand I 



0.7 



0.8 



0.9 



Figure 2-4. PCE Solubility Prediction of the Hildebrand and Extended-Hildebrand 
Theories for the IPA Cosoivent Mixtures. 



and small heat eflfects (Amidon et al. 1974). Alcohols are known to be associated through 
hydrogen bonding in the liquid state, with this association decreasing in order of primary, 
secondary, and tertiary due to steric limitations (Franks and Ives 1966). Other methods to 
improve the solubility prediction of alcohol systems, such as the Molecular Surface Area 
approach and the Microscopic Surface Tension (Amidon et al. 1974) have shown only 
"good" results. 

Williams and Amidon ( 1 984b) described the non-ideality of an ethanol-barbital- 
system at low- volume fractions of ethanol as due to greater solute-solvent interactions 
than solvent-solvent. This results in solubilities below ideal predictions. Conversely, at 



35 

high volume fractions solvent-solvent interactions dominate to result in above ideal 
solubility predictions (Williams and Amidon 1984b). This is exactly what occurs in 
ethanol and isopropanol cosolvent systems. 
Minor co solvent addition 

Results from addition of other less-polar solvents (in small fractions) to try to 
increase the solubility of PCE, while decreasing the total amount of solvents in the 
mixture, is presented in Figure 2-5. The benefits of this are little to none at all. For 
example, the solubility of PCE in a 60% EtOH/30% H2O/10% isobutyl alcohol mixture is 
just under 100,000 mg/1. This is a total alcohol content of 70%. A cosolvent mixture of 
only 70% EtOH/30% H2O results in a PCE solubility of approximately 90,000 mg/1. 
Furthermore, the addition of a less-polar solvent may not aid in solubility due to its 
partitioning into the DNAPL phase. Here, it does little to improve the aqueous solubility, 
but it can cause density changes if significant amounts of solvent partition into the NAPL 
phase. This partitioning can also cause swelling of the NAPL, possibly mobilizing 
DNAPL due to lower IFTs. These lower IFTs are due to the interface of these systems 
becoming surroimded by like molecules in both phases, reducing the tension between 
them. Even if there is a slight improvement in the solubility of PCE, the increased 
environmental risk of the addition of two solvents to the subsurfece and the added 
complexity of phase behavior and mobilization possibilities due to reduced IFT are not 
sufficiently outweighed. 



36 



1000000 



100000 



O) 

E 



3 

o 

CO 

UJ 



10000 



1000 



100 





y' 






H.,v' 








g 










■■ ■'' 


1 


B 

X 


X 












♦ 


X 
















o 


• 










- . 




o 

A 


e 












+ 
o 


D 


8 














1 






- ■ ' , 










X 

1 



















10 20 30 40 50 

% of Major Cosolvent 



60 



70 



80 



90 



• EtOH only x EtOH only (2) ■ IPA only O EtOH w/5% IPA + ElOH w/1 0% IPA 

♦ EtOHw/5%IBA O EtOH w/1 0%IBA A EtOH w/5% POH D EtOH w/5%P0H (2) 



Figure 2-5. Solubility of PCE as a fimction of various cosolvent volume fractions (initial 
phase volumetric phase ratio 1:1) 



Interfacial Tension Measurements and Predictions 

Interfacial tension measurements agree well with literature values (ImhofiFet al. 
1995; Pennell et al. 1996b). IFT exponentially decreases as a function of cosolvent 
volume fraction (Figure 2-6). IPA IFT measurements indicate that these mixtures have a 
stronger response to increases in volume fraction of cosolvent, as compared to the EtOH 
mixtures. In addition, over the ranges of economical remediation application (>70% for 
EtOH), IFT is fairly insensitive to additional volume fractions of cosolvent. If 
mobili2ation is a concern, then a drastic reduction in IPA fraction may be necessary, with 
resulting decreases in solubility being the tradeoff for hydraulic stability. Assuming the 



37 




• EtOH ■ IPA 



Figure 2-6. Relationship of equilibrated interfacial tension of PCE/alcohol/water ternary 
systems as a function of initial cosolvent volume fraction. 



appropriate regime is considered, a possible benefit of using IPA over EtOH may be that 
similar levels of solubilization may be achieved for smaller volume fractions of cosolvent 
(IPA vs. EtOH). 

A relationship between the logarithm of IFT and/c was determined and plotted as 
Figure 2-7. Data are strongly correlated, with coefficient of determination (R^) values of 
0.9978 and 0.9975 for EtOH and IPA, respectively. It is interesting to note that this 
correlation involves the volume fraction of cosolvent prior to mixing. This volume 
fraction is obviously not the same value after equilibrium has been achieved, especially for 
alcohols that can significantly partition into the NAPL phase, such as IPA. Although this 



38 



100 



0.1 



0.1 



y = 37.193e 
R^ = 0.9978 



3 603X 




0.9975 



0.2 



0.3 



0.4 



0.5 



' cosolvent 



0.6 



0.7 



• EtOH ■ IPA ♦ EtOH+10%IBA 



0.8 



0.9 



Figure 2-7. Logarithmic plot of the IFT of ternary PCE/cosolvent/water mixtures versus 
initial volume fraction of cosolvent. Additional data for addition of 10% isobutanol is 
shown for reference. 



relationship may have a weak scientific basis when using the pre-equilibrated volume 
fractions, the dependence is adequate to use as a predictive tool for field applications. This 
relationship is similar to the log-linear solubility relationship as represented by: 



lnIFT = Q/;+lnIFTo 



(2-18) 



where Q is now the "IFT reduction power" of the given cosolvent in aqueous mixtures 
and IFTo is the interfacial tension between pure water and NAPL (PCE). By regression of 
the data, QEtoH= -3.60 and Qipa= -5.80. This similarity to the log-linear relationship 



39 

should not be surprising because IFT is strongly dependent on the mutual solubilities of 
the two phases' solutes. -.' ' " 

Upon correlation of solubility and IFT (a combination of Figure 2-2 and Figure 
2-7), the results indicate a possible method for estimation of in-situ IFTs based upon 
solubility of PCE in the cosolvent mixture. This assumes local equilibrium is achieved. 
Figure 2-8, showing the logarithm of IFT as a function of the logarithm of the solubility of 
PCE, is the result. The largest deviations from a linear relationship occur at very low 
solubilities, where remediation technologies are not economically realistic. At higher 
solubilities, the prediction is quite close to experimental values and IFT predictions are 
within a few percent. The benefit of using such a plot is the direct estimation of in-situ 
IFT at the flushing front using the aqueous phase concentration of the given contaminant 
determined from extraction wells. Knowledge of this in-situ IFT is critical to determine 
the amount of mobilization that is likely occurring. This can be determined by using 
relationships developed by Pennell et al. (1996). 

Conclusions 

Use of log-linear solubility relationships is not a completely accurate method to 
predict solubility of PCE in cosolvent mixtures over the entire range of possible volume 
fractions. Improved predictions are possible at higher volume fractions of cosolvent. 
These predictions may be adequate for estimations necessary for field studies or 
remediation efforts. Deviations from the log-linear model are similar to those foimd in the 
literature (Dickhut et al. 1991; Li and Andren 1995) and can be explained by fimdamental 
theories described in the literature (Franks and Ives 1966). For improved estimation of 



40 



E 

I 



100 1 


• 


20^ 


^>>J""~"~--~^ 40 
























R^ = 0.963^^^ 


■~n40 


- 




60 


60 


R^ = 0.9703 




■■■---. 










* 


; '*"* 


^.''' 








-> 




ni - 






•- i 


■ -i 






Ij 











100 



100000 



PCE Aqueous Phase Solubility (mg/l) 



B EtOH ♦ IPA 



1000000 



Figure 2-8. Interfacial tension of PCE/cosolvent/water mixtures related to solubility of 
PCE in the aqueous phase. Numbers above selected data points indicate initial volume 
fraction of cosolvent. 



PCE solubilities, the use of the Extended Hildebrand or the UNIFAC model is 
recommended. Their added complexity is beneficial to accurate solubility predictions over 
the entire range of cosolvent fractions. 

The interfacial tension resulting from various cosolvent mixtures based on the 
initial volume fraction of cosolvent leads to an interesting relationship, which is similar to 
the log-linear model. An "IFT reduction power" can be determined for each cosolvent, 
which quantitatively describes the ability of the cosolvent to reduce the IFT as it is added 



■ ,. >■;;'■ ■ ■ " ^. 41 

in increasing volume fractions. IFT can also be accurately estimated by PCE aqueous 

phase solubility, especially in regimes conducive to cosolvent flushing. Due to the 

dependency of PCE aqueous phase solubility upon the aqueous and DNAPL phase ratio, it 

should be clarified that this approach is limited. Incorporating this predictive information 

into a trapping number relationship (Pennell et al. 1996b) will allow better prediction of 

regimes with solubilization, yet without mobilization of the NAPL/DNAPL phase. This is 

the topic of the next few chapters. 

A historical conclusion remains appropriate: 

"The best advice which comes from years of study of liquid mixtures is to 
use any model in so far as it helps, but not to believe that any moderately 
simple model corresponds very closely to any real mixture" (Scatchard 
1949) 



J. \^Jt i 



CHAPTER 3 
MOBILIZATION OF RESIDUAL PERCHLOROETHYLENE DURING COSOLVENT 

FLOODING 



Introduction 

Until recently, remediation technologies for the removal of organic contaminants 
from subsurface environments focused on the pumping of groundwater and subsequent 
treatment of these streams. Risk reduction to possible receptors was the driving force 
behind these actions. However, due to solubility limitations, remedial action time-scales 
are long and expensive for such treatment. Sources of contamination are very slowly 
removed due to natural solubilization. In the last few years, research efiforts and 
technology demonstrations have become more focused on source removal. These include 
surfactant flooding and cosolvent flushing (Annable et al. 1996; Falta et al. 1997; Fortin et 
al. 1997; Jawitz et al. 1998b; Lunn and Kueper 1997; Pennell and Abriola 1996; Pennell et 
al. 1994; Pope and Wade 1995; Rao et al. 1997). Although these techniques tend to be 
more aggressive and have higher initial costs, the removal of a long-term source is 
beneficial from risk reduction, economic, and legal perspectives. 

Of these recent technologies, methods that increase the solubility of the 
contaminant into a mobile flushing phase have shown promising results (Annable et al. 
1996; Fountain et al. 1991). Two general types of chemicals are used to enhance 
contaminant solubility: surfactants and cosolvents. Both increase the aqueous phase 
solubility of the contaminant by up to five orders of magnitude, thereby accelerating 

'f« '■' ;■> , -. 

42 



43 

remediation efforts. The resulting faster cleanup times are desired to decrease health risks 
to potential receptors and to reduce project operations and maintenance costs (Sillan 
1999). 

These processes also reduce the interfacial tension between the aqueous and 
organic phases. This reduction can drastically change the force balance keeping the 
organic phase trapped in the soil pores rather than being forced out due to the advective 
flow of the flushing phase. This possible movement of the organic phase has been labeled 
'mobilization'. Correspondingly, the residual NAPL left behind after any flushing action 
designed to reduce the NAPL saturation is labeled as being 'entrapped'. The process itself 
is termed 'entrapment'. Literature related to these processes is large, yet incomplete in 
many aspects, since the mechanisms are complicated by interrelated properties, including 
complex formation pore structure, fluid properties, and applied conditions. In addition, 
the variability of the media and fluids is so great that most generalized conclusions have 
limited applicability (Stegemeier 1977). 

Mobilization of oil for the purpose of Enhanced Oil Recovery (EOR) has been 
studied for a number of years, by several research communities (Lam et al. 1983; Moore 
and Slobod 1956; Morrow 1987; Morrow et al. 1988; Morrow and Songkran 1981; Patel 
and Greaves 1987; Ramamohan and Slattery 1984; Taber 1969). This research focused 
primarily on the use of surfactants to decrease the interfacial tension and mobilize the 
entrapped oU phase, with eflSciency increased by use of a polymer flood behind this bank. 
In feet, the first patent issued to cover the use of surface-active materials as an aid to the 
water flooding of petroleum reservoirs was awarded in 1927 (Atkinson 1927). This 
concept and the conclusions resulting fi-om the associated research has been more recently 



44 

applied to the field of surfactant and cosolvent flushing (Annable et aL 1996; Augustijn et 
al. 1997; Pennell et al. 1996b). Taber (1981) recognized the tendencies of the research 
and oil recovery communities to use high quantities of alcohols as "cosurfactants" in 
flushing formulations. He noted that although alcohols are expensive, "the potential 
advantages for oil [or NAPL] recovery are so great that future research may continue to 
examine the possibility of using alcohols as the main slug material for some processes." 
Earlier research applied to EOR focused on the relationships between viscous forces of the 
flushing fluid and the capillary pressures associated with holding residual oil in the pore 
structure (Stegemeier 1977; Taber 1969; Taber 1981). Later research amended these 
relationships to include not only viscous and capillary forces, but forces associated with 
buoyancy effects (Morrow et al. 1988; Morrow and Songkran 1981; Ng et al. 1978). In 
most historical research on this topic, buoyancy forces were neglected, or the phases 
chosen so that their phase densities were nearly identical (Pennell et al. 1996b). These 
buoyancy effects can become significant as density differences between phases become 
large, especially when applied to chlorinated hydrocarbon contaminant systems. These 
mobilization and entrapment relationships developed will be defined below. 

Attempts to change the balance of forces and permit an aqueous flushing phase to 
release or displace a NAPL effectively may be classed into three broad and often 
overlapping categories. These are attempts to (1) change wettability, (2) change oil- water 
interfacial tension, or (3) remove the interface completely via miscible flooding (Taber 
1981). The interplay of each of these processes is so great during cosolvent flooding that 
this operation cannot be put solely into one or the other category. However, any 
cosolvent remediation scheme employed today can be classified via the main NAPL 



45 

displacement process desired. These are either complete solubilization or mobilization of 
the contaminant. This is not to say that the secondary process is avoided at all times. 
Again, the forces that are in action during these operations do not allow such segregation. 
For the sake of discussion purposes, these two categories are used below. 
Solubilization vs. Mobilization 

Increased solubilization of contaminants occurs when the aqueous phase becomes 
more similar in polarity to the organic phase. When two phases are dissimilar in polarity, a 
tension develops at the interfece causing the two phases to remain separate. As modifiers 
(such as cosolvents) are added to the system, the two phases become more similar, 
sohibilization is increased, and the interfacial tension (IFT) is reduced. If enough modifier 
is added, the IFT can be decreased to very low values, and ultimately to zero, at which 
point the two phases are miscible. It is very low IFT regimes where mobilization of the 
organic phase can result (Pennell et al. 1996b). This is because the rate of solubilization 
may not keep pace with the lowering of IFT, resulting in high, mobile DNAPL saturations. 
The excess fi-ee phase organic can now move as a separate phase xmder the reduced IFT. 
If the organic contaminant is denser than water (DNAPL), such as perchloroethylene 
(PCE), mobilization can lead to movement of contaminants to deeper aquifers. Hydraulic 
controls during remediation may reduce the chance of downward mobilization. 

In some instances, mobilization of the NAPL is favored, especially for an LNAPL. 
However, a question developing in the field of in-situ flushing of DNAPLs, is the whether 
to flush under cosolvent (or surfactant) conditions which encourage mobilization of the 
DNAPL plume or simply to enhance solubilization of the DNAPL contaminant into the 
flushing mixture. In some situations, one may be more favored over the other. Predictive 



46 

capabilities allowing engineers to better understand the regime in which they desire to 
remediate would be beneficial. An improved understanding of what occurs at the 
transition between solubilization and mobilization regimes is desired. 

Remediation of residual NAPL by contact with a flushing alcohol-rich solution is a 
complex process. During cosolvent flooding both solubilization and mobilization of the 
NAPL can occur. Mobilization occurs due to a variety of hydrologic and physical 
parameters. A NAPL globule is displaced when the IFT is reduced to an extent that the 
forces created by the presence and motion of the continuous aqueous phase and buoyancy 
is sufficient to overcome capillary forces holding the NAPL globule in place (Lam et al. 
1983). In studying the process of mobilization, complexities arise because the trapped 
NAPL phase and the cosolvent containing aqueous phase are not in chemical equilibrium 
and mass transfer occurs fi-om one phase to the other. Hirasaki (1980) has discussed some 
of the many non-equilibrium phenomena that can contribute to the mobilization process 
(Lam et al. 1983). To isolate the effects of reduction of IFT on mobilization of PCE, soil 
column experiments were conducted with the influent cosolvent phase equilibrated and not 
equilibrated with PCE. Furthermore, solubilization of a partitioning cosolvent such as t- 
butyl alcohol (TBA) into the NAPL can cause density reduction; hence, a volumetric 
swelling of the NAPL. Severe swelling in itself may cause mobilization (Lam et al. 1983). 
This will specifically be addressed in a later chapter. 

While interfacial tension (IFT) is critical, it is not the only parameter governing 
mobilization. Relationships have been established and applied to surfactant use in porous 
media (Pennell et al. 1996b). These relationships describe the amount of NAPL removed 
(or remaining NAPL saturation) fi-om a given media via mobilization as a fimction of a 



47 

dimensionless "trapping number", which includes contributions from viscous, capillary, 
and buoyancy forces, as described below. 
The Trapping Number Relationship 

To illustrate the interplay of viscous and buoyancy forces on the displacement of 
an organic liquid in two-dimensional domains, the relationship of the trapping number 
developed by Pennell et al. (1996b) can be used. Other authors have arrived at similar 
relationships, which linearly combine a "capillary number" and a "bond number" (Dawson 
and Roberts 1997). The Pennell study investigated the influences of forces on the 
mobilization of residual PCE during surfectant flushing. The balance of forces was in 
terms of two dimensionless numbers - the capillary and Bond numbers. 

The capillary nimiber is defined in terms of aqueous flow within a pore, and relates 
the viscous to the capillary forces: 

^'- No.=-^^ . (3-1) 

where q„ [LT"'] is the Darcy velocity of the aqueous phase, fiy/ [ML't'] is the dynamic 
viscosity of the aqueous phase, 7^^ [MT^] is the IFT between the organic liquid and 
water, and 6 is the contact angle between the NAPL globule and the pore wall (usually 
assumed to be zero for low IFTs situations). 

The bond number represents the ratio of buoyancy to capillary forces. It is 
represented by 



. 15? O'i, * 



48 



N.'^^ (3-2) 

where Ap is the density difference between the two liquids [M L'^], g is the gravitational 
constant \LT\ it is the intrinsic permeability of the porous medium [L^], and krw is the 
relative permeability for the aqueous phase. 

A total trapping number {Nj) was developed that relates viscous and buoyancy 
forces to the capillary forces acting to retain organic liquids within a porous medium 
(Pennell et al. 1996b). For vertical flow, A^^ ^ ^^^ sum of the two dimensionless numbers, 
the capillary number (TV^,^) and the bond number {N^: 

Nr-/Nc, + N^. '^ - (3-3) 

In the case of horizontal flow the trapping number is: 



N,=4nI+NI . (3-4) 



When the trapping number is exceeded, the combination of viscous and buoyancy 
forces exceeds the capillary forces holding the NAPL globule vdthin a given pore. This 
excess force will cause the globule to physically move through that pore. In Permell's 
(1996b) laboratory studies, there is not a sharp point when mobilization begins, but rather 
a sloping curve when PCE saturation is plotted against the logarithm of the trapping 
number. This is due to small anisotropics within the "homogenous" sand columns used 
(Pennell et al. 1996b). Researchers have observed that smaU-scale heterogeneities might 
lead to locally high residual DNAPL saturations that are more easily mobilized than 
DNAPL residuals in homogenous media (Imhoff et al. 1995; Padgett and Hayden 1999). 



49 

As demonstrated by Pennell et al. (1996b), these relationships can be used to 
predict the soil, hydraulic and IFT conditions required for the onset of PCE mobilization. 
Their study using surfactants indicated that ultra-low IFTs (<0.001 dyne/cm) are not 
required to induce mobilization of PCE in unconsolidated porous media. Therefore, 
predictive capabilities in low IFT ranges (0.1 to 10 dyne/cm) would be beneficial, when 
considering cosolvents. Their resuks indicate that the value ofNr should be less than 2 x 
10 to minimize the potential for NAPL mobilization. They also concluded that NAPL 
mobilization is a more eflScient recovery process than micellar solubilization. Finally, 
comparison of data fi-om Pennell et al. ( 1 996b) and historical data showed that the 
trapping number is applicable to systems with or without significant buoyancy effects. As 
mentioned previously, the Pennell study was conducted using surfactant solutions. To 
date, no reference has been found in the literature that has generated complete 
mobilization curves using cosolvents in simulated porous media. Padgett and Hayden 
(1999) used the same mobilization relationship. However, their focus was the onset of 
mobilization of PCE via ethanol flushing in varying heterogeneous media. It is critical to 
use the total trapping number analysis when selecting surfactant formulations to minimize 
NAPL mobilization (Pennell et al. 1996b), but it is proposed this can be extended to 
cosolvents as well. 

Study Objective 

The objective of this study was to conduct soil column experiments similar to 
Pennell et al. (1996b) using cosolvent mixtures typically used in the remediation field and 
predict mobilization characteristics of PCE using the trapping number relationship. 



50 

Comparison of mobilization curves to historical data is desired, as well as possible 
differences in surfactant versus alcohol systems, and finally, differences in cosolvents used. 

Materials and Methods 

HPLC grade PCE (CAS 127-18-4) and isopropyl alcohol (CAS 67-63-0) was 
obtained fi-om Fisher Scientific, Fair Lawn NJ. The absolute ethanol (>99.5 %; CAS 64- 
17-5) used in these studies was purchased through Spectrum Quality Products, Inc., 
Gardena CA. The water used for the cosolvent solutions and for soil column flushing was 
purified through a Nanopure™ filtration process, and brought to an ionic strength of 10' 
M (350 ppm) with calcium chloride. This is published as an average groundwater ionic 
strength value (Stumm and Morgan 1981). Stock solutions of cosolvent/water mixtures 
were made with varying volume fi-actions of cosolvent. These solutions were made in 1- 
liter quantities using standard volumetric glassware. 
GC Analysis 

Component solution concentrations were determined via gas chromatography 
(GC) analysis. GC analysis was performed on a 30 m x 0.530 mm, 3 ^m fixed phase, DB- 
624 column, manufactured by J&W Scientific, using a flame ionization detector (FID). 
Although the detection limit for PCE is much lower for an electron capture detector 
(ECD), ultra-low (ppb) detection was not required for this study as the lowest expected 
value was the solubility of PCE in pure water (150 ppm). Additionally, the strongly linear 
response of the FID over several orders of response magnitudes made it the desired 
choice. 



51 

Physical Measurements 

Density measurements were performed gravimetrically. Two milliliters of solution 
were measured in a gas-tight volumetric syringe and weighed on a precision Mettler 
Balance (± 0.0001 g). A sample's density measurements were repeated at three times to 
ensure accuracy and precision of this technique. Viscosities of solutions were determined 
by a Cannon-Fenske Routine Viscometer (Cannon Instrument Company, State College, 
PA). A du Nuoy ring tensiometer (Fisher Tensiomat Model 11) was used to determine the 
equilibrium interfacial tension of all samples. The lower limit of this instrument is 
approximately 0.1 dyne/cm, although IFTs below 1.0 dynes/cm are subject to visual and 
experimental error. These values were used in all trapping number calculations, assuming 
equilibrium is quickly achieved within the soil column. For strongly partitioning alcohols, 
this assumption becomes less valid. This method will tend to underestimate the IFT and 
therefore overestimate both the capillary and trapping numbers, since the nonequilibrium 
IFT is greater than the equilibrium value (Lam et al. 1983). 
Sand Column Preparation 

A small-scale glass column (4.8 cm x 15 cm, chromatography column from Kontes 
Corporation) was used for this study. All end materials shipped with the column were 
removed except for the 30-40 mesh nylon screen. The soil column was incrementally 
packed with well-sorted Number 30-40 sand. This sand size was chosen so that the pore 
size would be approximately equal to the screen mesh size to avoid entrapment of NAPL, 
yet the sand still contained within the colimin. Vibration of the soil increments was also 
performed to improve packing characteristics. Once the column was packed it was 
weighed with all necessary column parts attached. The soil mass was weighed by 



52 

difference and the internal volxime of the column used to calculate the bulk density. Using 
the density of silica sand (2.65 g/cm^) and the mass of sand added to the column, the 
vohime of sand (Vg) can be calculated. The porosity of the soil column is then easily 

calculated form the total volume of the column as: t) = {l-W^fVi. Approximately 15 

pore volumes of de-aired water (via vacuum) were then pumped through the column and 
the pore volume determined. = . 

PCE Saturation 

"Pure" PCE (dyed with 5 X 10-5 M oU-red-o dye, Fisher Scientific, CAS 1320-06- 
5) was introduced to the column to establish residual saturations. This dye concentration 
has been shown not to significantly affect solubilization and IFT properties (Pennell et al. 
1996b; Young 1999). The PCE was introduced in an up flow mode to achieve stable 
displacement of water. When PCE appeared at the top of the column, the flow rate was 
increased 5 fold to increase PCE saturation (Dawson and Roberts 1997). The flow was 
then reversed and 3 pore volumes of water pumped through in a down flow mode to 
displace fi-ee product PCE, at a flow rate of 5.0 ml/min. The flow was again reversed and 
a few milliliters of water pumped into the column to remove PCE held at the influent 
screen due to end effects. The resulting PCE saturation (%S'^f) was determined 
gravimetrically. 
Hydrodynamic Parameters 

The intrinsic permeability (k) of the porous media was determined by measuring 
inlet and outlet pressures during aqueous phase flow. Resistance due to column end 
effects and tubing were measured independently using an empty soil column of the same 



A ^il 



53 

construction and identical tubing and fitting (Morrow et al. 1988). This resistance was 
subtracted from pressure drop measurement over the filled column to determine pressure 
drop across the media only. A differential pressure transducer (Cole-Palmer Instrument 
Company, Niles IL, 0-5 inches H2O differential transducer) was used to monitor this 
pressure difference at various times during an experimental run. 

Relative permeability (Ar) values were determined again by differential pressure 
measurements at various DNAPL saturations. However, these measurements during 
initial runs were inconsistent. Subsequently, all relative permeability values were 
estimated using van Genuchten parameters (van Genuchten 1980) for the sand medium, 
found from Tempe cell testing. 
Sand Column Mobilization Studies 

Experiments were conducted, similar to Pennell's (1996b), to develop a 
mobilization curve for PCE and cosolvent mixtures. The column was sequentially flushed 
with increasing volume fractions of cosolvent, continuously. Gradually increasing inlet 
cosolvent fractions avoided front instabilities due to the density differences. At the front, 
due to dilution, PCE may come out of the flushing phase, creating a macroemulsion. This 
emulsion may resolubUize as it is exposed to the higher cosolvent fractions, or elute from 
the column as a macroemulsion. This is not desired, as this quantity of PCE is more 
difficult to quantify. Gradient elution was performed, also in part, to help minimize 
macroemulsion formation. 

To determine the amount of PCE solubilized compared to the amount mobilized, 
one of the phenomena must be eliminated to quantify both. The trapping number curve 
was first constructed using aqueous streams (water plus cosolvent) equilibrated with PCE. 



54 

This eliminated solubilization and allowed visual volumetric determination of mobilization 
(from purely IFT reduction) based on the PCE phase generated from the column. Similar 
experiments were then conducted on the same sand colimin using non-equilibrated ethanol 
mixtures (without any PCE added). PCE saturations and trapping numbers were 
determined and results between the two methods compared. Equilibrated cosolvent runs 
were then repeated using IPA and t-butyl alcohol as cosolvents and compared to those of 
ethanol to investigate swelling impacts. 

For each cosolvent fraction, the run was continued for at least one pore volume, 
generally two, to ensure the resident fluid was characteristic of the injected fluid, yet 
minimize any possibility of local solubilization. Gradient elution improves the eflSciency of 
this process. The remaining NAPL saturation percentage (%Spce) was then determined 
both gravimetrically and volumetricaUy (based on visual measurement in a graduated 
cylinder). This was done for various cosolvent volume fractions ranging from 20% to 
90% v/v cosolvent/water mixtures. 

Results and Discussion 

Equilibrated Gradient Column Studies 

For each run, a series of trapping numbers (Pennell et al. 1996b) was determined, 
using the predicted IFTs from the batch equilibrium experiments. A plot of %Spce versus 
trapping number, Nt , was then generated. Results from gradient soil column 
displacement experiments are shown in Figure 3-1 through Figure 3-3. Figure 3-1 is the 
effluent PCE concentration as a fimction of pore volumes (PV; 1 PV is approximately 100 
ml) of saturated EtOH/HaO/PCE cosolvent mixtures flushed through the column. Note 



55 

that the volume percentages of ethanol shown are pre-equilibrated volume fractions, which 
differ from equilibrium volume fractions, especially at higher percentages of ethanol. It 
can be seen that eflQuent PCE concentrations, after 1 PV of each fluid has passed, 
approach equilibrium conditions. Significant mobilization begins to occur when the 85% 
EtOH solution is resident within the column. Reduction in DNAPL satiiration at earlier 
pore volumes (0-4 PVs) is thought to be artificial, caused by small amounts of PCE being 
removed from the effluent end of the column apparatus due to possibly lower capillary 
forces, under moderate IFT reductions. 
Blank Equilibrated Gradient Study . =. • 

A gradient experiment was conducted virtually identical in procedure to the one 
described above, except no PCE was loaded into the column. Each flushing phase was 
pre-equilibrated with PCE. This was done to determine if any of the free-phase PCE 
generated from the cokmin during any experiments could arise from simply the dilution of 
solubilized PCE at each of the gradient fronts. After completion of the entire gradient, 
less than 0.2 ml of PCE was collected. This volume was decided to be insignificant to our 
studies. The possibility of frontal dilution contributing to the mobilized DNAPL volume 
was discarded. 
Non Equilibrated Column Studies 

Results from a non-equilibrated gradient elution are presented in Figure 3-2. 
Effluent concentrations show that these mixtixres at study flow regimes quickly reach 
equilibrium conditions. Mobilization does occur as shown, but it represents a very small 
percentage (< 0.7%) of the total DNAPL saturation. Under this gradient regime, 
essentially all DNAPL was removed via solubilization prior to introduction of the 90% 



56 



000000 1 








90% .„„ ^ , . 


■- 


1 




85% 1 — .^******** * 

80%^ ^«<.*' 








PCE influent 


60% 


♦ * 


- 






••< 




* 






40% 


* 


A 


- 






• 




20% 


••«• • 


i,' 


■ 


100 - 


•• 
• 
• •- 







CO 

a. 

80 < 
O 



0.0 10 2.0 3,0 



4.0 5.0 6.0 

Pore Volumes 



7.0 8.0 90 10.0 



Figure 3-1 . Gradient effluent profile for saturated PCE run (influent %'s shown are 
ethanol volume fi^actions prior to saturation). 



S 10000 

£, 

c 
o 

1 



Ul 

g 100 











85% 


90% 






■ 


. ...... 










60% 


- ./ '^ 




'Solubility Potential' of Influe 
\ 


It 


j 






\ 40% 




A 


I 




i 


/ 


20% 


I 




A 


\ 


«er "t • e > i e •' 








A 




A 
A 
.A 


\ 




..--• 


" 



0.5 I 
1 

04 g 
I 



3.0 4.0 

Pore Volumes 



Figure 3-2. Gradient effluent profile using unsaturated ethanol mixtures - percent of 
mobilization shown. 



57 




3.0 4.0 

Por* Volumes 



Figure 3-3. Gradient eflQuent profile using unsaturated ethanol cosolvent mixtures with 
PCE saturation reduction shown. 



EtOH cosolvent. Significant reductions in PCE saturations occur during the injection of 
the 80% EtOH mixture. This can be seen in Figure 3-3. Calculations to determine the 
trapping number were conducted using physical measurements shown in Table 3-1. 



Table 3-1. Physical Measurements of PCE Saturated Cosolvent Solutions 

Kinematic Dynamic 
EtOH IFT As Viscosity viscosity 



v/v % dyne/cm g/cm 



cSt 



cP 






37 


1.002 


0.92 


0.922 


20 


15.85 


0.9752 


1.586 


1.547 


40 


7.74 


0.9444 


2.42 


2.285 


60 


4.25 


0.9107 


2.542 


2.315 


80 


1.91 


0.9303 


1.945 


1.809 


85 


1.14 


0.9689 


1.661 


1.609 


90 


0.55 


1.0765 


1.285 


1.383 



:■■■» ),: 



58 

Generation of Mobilization Curves 

Relative permeabilities {kr) calculated from pressure measurements made during a 
run were inconsistent due to probable variability in column conditions, including strong 
buoyancy effects. Therefore, permeabilities during the run were estimated using van 
Genuchten parameters (van Genuchten 1980) for the sand medium, found from Tempe 
cell testing. Although these values are calculated, they are reasonably close to actual 
values. Furthermore, small differences in kr will not introduce significant error into the 
trapping number. The contact angle (0) for the relationship was assumed to be zero. 
Although this is probably not valid at higher IFT values, it becomes more appropriate as 
IFT decreases, and subsequently in areas where mobilization occurs. Data points to 
construct these curves were based on properties of the displacing fluid and relative 
permeability of the media being flooded. As can be seen in Figure 3-4, mobilization for 
PCE begins at a trapping number of approximately 2 x 10"^. This is different by an order 
of magnitude from that of Pennell (1996b). For surfactants, it was predicted that 
mobilization of PCE would begin at a trapping number of approximately 2 x 10'^ to 5 x 

10-^ • . 

Also shown in Figure 3-4 is the PCE desaturation curve for the non-equilibrated 
run. This clearly shows the non-equilibrated experiment never reaches the critical trapping 
number required for mobilization. On a column average basis, the saturation decreases 
due to solubilization before the mobilization trapping number is reached. Therefore, 
significant mobilization in the column effluent is not observed. Additionally, the gradient 
profile could have been stopped, and the injected concentration fixed at any one of the 
alcohol fractions (60, 80 or 85%) and saturations reduced to zero without 



59 




1.0E-05 



1.0E-04 



1 .OE-03 



Trapping Number 



igradient2 X9radient4 ♦non-equilibrated gradient 



Figure 3-4. Mobilization curves showing effect of a cosoivent (ethanol) flushing phase 
which is pre-equilibrated with PCE (gradient 2 and 4) and a flushing phase with foil 
solubilization potential (non-equilibrated). All are gradient runs. 



mobilization. This can be seen if one extrapolates the portions of the desaturation curve 
where significant reduction in saturation occurs. Thus, if flooding regimes are controlled, 
the removal process of NAPL may never cross the mobilization envelope. 

Resulting mobilization curves for the ethanol system are shown in Figure 3-5 with 
Pennell et al. (1996b) data shown for reference. The data indicate that mobilization of 
PCE begins at a trapping number of approximately 2 x 10"^. Three gradient runs are 
shovra in addition to three riins that were conducted independently, without any gradient. 
These were conducted to verify that the desaturation curve for PCE residual was not 
dependent on mode of flushing or previous exposure to lower cosoivent volume fractions. 
As can be seen, the trapping relationship is independent of the mode of flushing. 



60 



18 



16 



12 



E 



S 

UJ 



10 



8- 



6- 



1 I 



! n X 



i ■ \ 



\ ! t In 

! ! M i '^^ 

i ! hi I 



I 



I ! 



I 1 1 i MH 
i I 



; I 



I 



■ I 






i I i i 



: i Mi 



• ! 



I 
1 



i 1 



i i i 



; M M M 

' i ■ I ? I i 
I i ■ ! n i 



i j I ! I i 
• i M 



•l 



i I M ii 

'■ i I iiilji 





i ■ i * 


k^ 


! !■ 


i ■ 


1 . 

i 








1 1 


j 


;i- ^1 



1.0E-06 



1.0&05 



1.0E-04 

Trapping Number 



1.0&03 



APennell (1996) » gradients ■ gradient4 ♦gt3dient5 * Single Runs 



1.0BO2 



Figure 3-5. PCE Desaturation Curves - PCE saturated ethanol cosolvent runs compared 
with data from Pennell et al. ( 1 996). 



From initial results, it appears that there is a difference between this study's data 
and those from the surfactant work of Pennell et al. (1996b). Taber (1969) noticed a 
difference between displacements of residual oil with surfactants and water/alcohol 
systems. Although the Taber' s initial critical value of the capillary number only was 
approximately the same in each case, the surfactant tended to desaturate more oil for the 
same capillary nimibers, i.e., more oil was recovered at a lower capillary number. Taber 
explained this difference by the adsorption of surfactant on the media surfaces, causing 
earlier mobilization due to lower interfacial forces. Pennell et al. (1993) noted, that 
critical trapping number values derived from the capillary and Bond numbers is system 



61 

specific and can vary over an order of magnitude depending on the properties of the 
organic phase, matrix, and experimental design. To determine the possible reasoning for 
this difference, surfactant solutions similar to Pennell et al. (1996b) were made and their 
methods repeated. The results are shown in Figure 3-6. Physical data of each surfactant 
solution were assumed to be those published in Pennell et al. (1996b). Spot checks of 
solution properties matched those in their work reasonably well, but values of IFT were 
getting too low (<1 dyne/cm) to be reproducible with the du Nuoy ring tensiometer used 
for this work. The data fi-om the surfactant series falls essentially on top of the cosolvent 
grouping. This suggests that the difference between the data sets is not due to differences 
between surfactant and cosolvents, but rather those related to media or experimental 
specifics. •[ ' 



18 



It' 



a 12 

c 

c 
■ 

I 10 



« 

CO 

111 
O 
0. 



2 



i ! ■! 



I I 
j ; 

Iji ■ 



i M ! ! ; i ! 



i ■•;« 



! [ X 






A 
I i ! ! i i ! 



i |i 






I i 



1 OE-04 

Trapping Number 



I A Pennell (1996) ■ gradient2 > graclient4 ♦ gradients » Single Runs o Surfactant 1 1 



Figure 3-6 Ethanol mobilization curves with surfactant run superimposed. 



62 

To further understand possible differences between NAPL removal due to 
solubilization and mobilization, runs were conducted with all phases in equilibrium with 
each other. The only column loading and flushing method possible to achieve this was to 
first load the column with the desired cosolvent that had been pre-equilibrated with PCE. 
The corresponding equilibrated PCE phase was then loaded into the column (up flow) at 5 
ml/min until PCE was eluting fi-om the top of the column, then flow was increased to 25 
ml/min until a total of one pore volume of DNAPL had been introduced. Equilibrated 
cosolvent then was flushed downward through the column (down flow) at 5 ml/min to 
bring the DNAPL phase to a new residual. This method and the results are described in 
the next chapter. 
Swelling Efifects of Cosolvents j 

Results fi-om similar experiments conducted for ethanol are shown in Figure 3-7 
and Figure 3-8 below for isopropyl alcohol (IP A) and t-butyl alcohol (TBA), respectively. 
Physical properties of these solutions are in Table 3-2. Swelling of the DNAPL due to 
IPA partitioning is slight. The impact of this swelling is not significant on the outcome of 
the onset of mobilization, as shown on the trapping number curve. Note that the volume 
of DNAPL remaining behind after each gradient increase in cosolvent volume fi-action had 
to be corrected back to a pre-flushing volume for comparison purposes. Swelling of the 
PCE due to TBA was great, making mass balance calculations subject to probable error. 
Swelling correction factors were based on batch studies using a 1 : 1 aqueous to PCE initial 
phase ratio. 



14 



» 12 



U 

a. 



X + 



y 



iliXx i 

«; . • I X 



i MM!! 

I i Mtii 
' ^ M: 



! i i i! 



o P 



io 



II 



! i 

i 1 



! : ■ I 



i ; I 



lilii 

iliil 

ill! 

H II 
a; i 

Hi 



] 

\ 

1 
1 


■lii 



! ,-. 



1.0E-06 



1.0E-04 

Trapping Number 



10E-03 



I A Pennell (1996) 9radient2 - gradient4 ■> gradients Single Runs • IPA Gradient + IPA Single Runs | 

Figure 3-7. Results from mobilization studies using pre-equilibrated IPA solutions, 
superimposed on the ethanol study results. 



18 



14 



a 12 

c 



I 10 



1 

3 

I 
Ul 

u 

Q. 



ill 

mil 

i 



■n 



i i -J 1 I I I 



; 9: 



X 

a 



1 I 



• 1 
! i 



-■ ;f 

Hi 



i 1 



! M i ii 
I illiil 



1.0E-06 



1 OE-Ot 

Trapping Number 



1.0E-03 



1.0e-Q2 



I A Pennell (1996) 9radient2 ■ gradient4 o gradients Single Runs ■ TBA Gradient 







Figure 3-8. Results of mobilization of PCE during gradient TBA column flushing; TBA 
pre-equilibrated with PCE. ■ ■ -<• 



64 



Table 3-2. Physical properties of solutions used in swelling mobili2ation studies. 

Kinematic Dynamic 
Cosolvent fc IFT pcos Viscosity Viscosity 

v/v % dyne/cm g/cm3 cSt cP 



. 


lOOVoHzO 


37.00 


1.002 


0.929 


0.930 


IPA 


40 


3.01 


0.947 


3.08 


2.91 


IRA 


60 


1.17 


0.931 


3.57 


3.32 


IPA 


75 


0.42 


0.975 


2.89 


2.82 


IPA 


85 


0.08 


1.087 


1.99 


2.16 


TBA 


19.2 


5.69 


0.978 


2.09 


2.05 


TBA 


32.1 


1.28 


0.969 


2.69 


2.60 


TBA 


49.6 


0.36 


0.960 


3.21 


3.08 



Conclusions 

The trapping number is an effective parameter to help predict mobilization of non- 
aqueous phase liquids in subsurface environments. Trapping number results and onset of 
PCE mobilization were found similar, although slightly greater, to those of 
Pennell et al. (1996b) for both surfactant and cosolvents. Ethanol used as a cosolvent (at 
volume fractions less than 85%) enhanced solubilization of PCE to the point where this 
process is dominant and mobilization of PCE can be avoided for the media studied. 
However, under severe conditions, mobilization using cosolvents can occur. This includes 
large step inputs to high cosolvent fractions, where DNAPL saturation is still great enough 
for immediate IFT reduction to cause mobilization, at least in a local sense. This of course 
could be important if, within that locality, DNAPL moves out of the zone of hydraulic 
control. These issues are fiirther addressed in two-dimensional box studies. 

As should be expected, differences between surfactant and cosolvent systems are 
not apparent on a mobilization curve. Mobilization curves appear to be independent of 



65 



alcohol type. Swelling effects, when DNAPL volumes are adjusted to pre-equilibrated 
values did not appear to affect onset of mobilization. However, as partitioning of the 
alcohol into the NAPL increased, the volume of NAPL increases and becomes difficult to 
quantify. Further research into this area is needed. 



CHAPTER 4 

ENTRAPMENT VERSUS MOBILIZATION OF RESIDUAL 

PERCHLOROETHYLENE DURING COSOLVENT FLOODING 

Introduction 

Enhanced Oil Recovery (EOR) has been practiced for quite some time and 
approaches have been "refined" to improve the collection efficiency of oil. In the oil 
recovery industry, quick and efficient removal of oil fi-om subsurface environments is 
obviously desired. EOR is achieved under immiscible conditions either by reducing the 
amount of oil entrapped or by mobilization of some of the trapped oil. Under strongly 
water v^^etting conditions, which is assumed throughout this research, trapped NAPL is 
held as discrete blobs. The processes of entrapment and mobilization are associated with 
displacement of continuous and discontinuous oil, respectively (Morrow et al. 1988). 
Therefore, maximizing mobilization of fi-ee-phase NAPL and minimizing the amount 
entrapped behind the flooding front is desired. For the remediation of contaminant plume 
sources, minimization of contaminant left behind is an obvious goal from a risk 
management standpoint, but if mobilization of banks of NAPL is the desired scheme, 
maintaining this bank by minimizing entrapment is also desired for process efficiency. 
Many studies have been conducted focusing on these processes relating to EOR (Moore 
and Slobod 1956; Morrow 1987; Morrow et al. 1988; Stegemeier 1977; Taber 1969). 
With the recent increase in application of this technology to remediation of contaminants, 
additional information relating to these processes 

66 



67 

specifically focused on NAPL contaminants is needed. Until recently, remediation 
technologies for the removal of organic contaminants fi-om subsurface environments 
focused on pumping of groundwater and subsequent treatment of this stream. Risk 
reduction to possible receptors was the driving force behind these actions. However, due 
to the solubility limitations of these types of treatment, remedial action time-scales are 
long and expensive. The source of contamination is very slowly removed due to natural 
solubilization. In the last few years, research efforts and technology demonstrations have 
become more focused on source removal. These include surfactant flooding and 
cosolvent flushing (Chaudhry 1994; Fortin et al. 1997; Pennell and Abriola 1996). 
Although these techniques tend to be more aggressive and have high initial costs, the 
removal of a possible long-term source is beneficial fi-om risk reduction, economic, and 
legal perspectives. 

Of these recent technologies, methods that increase the solubility of the 
contaminant into a mobile flushing phase have shown promising results (Annable et al. 
1996; Falta et al. 1997; Fountain et al. 1991; Jawitz et al. 1998b; Rao et al. 1997; Sillan 
1999). Two general types of chemicals are used to enhance contaminant solubility: 
surfactants and cosolvents. Both increase the aqueous phase solubility of the contaminant 
accelerating remediation efforts by two to five orders of magnitude. The resulting faster 
cleanup times are desired to decrease health risks to potential receptors and to reduce 
project operations and maintenance costs. 

These processes also reduce the interfacial tension between the aqueous and 
organic phases. This reduction can drastically change the force balance keeping the 
organic phase trapped in the soil pores or being force out due to the advective flow of the 



6$ 

flushing phase or density contrasts. This possible movement of the organic phase has been 

labeled 'mobilization'. 

Solubilization, Mobilization and the Trapping Number Relationship 

Prior discussion and literature review of solubilization and mobilization of NAPLs 
via cosolvent and surfactant flushing can be found in chapters 2 and 3 and is not repeated 
here for brevity. The reader is encouraged to review those sections, if necessary. 
Mobilization and Entrapment of Residual Non- Aqueous Phase Liquid 

Differences between the processes of entrapment and mobilization have been 
documented previously in EOR research (Morrow et al. 1988; Morrow and Songkran 
1981). During their entrapment experiments, saturations appeared uniform throughout the 
column and relative permeabilities at reduced residuals were not fiinctions of time and 
flow rate. Morrow and Songkran (1981) estimated that mobilization of trapped NAPL 
blobs is about five times more difficult to achieve than prevention of trapping. In their 
efforts to mobilize a trapped gas, severe solution ejects (due to pressure gradients and 
gas solubilities) were encountered in an attempt to mobilize by increasing the capillary 
number. These were in distinct contrast to trapping behavior, where solution effects 
proved to be insignificant (Morrow and Songkran 1981). During the entrapment process, 
local changes in interfacial shapes within individual pores are small and not likely to 
account for the large changes in residual saturation that were measured under different 
capillary numbers. The mechanism of entrapment, they believed, is due to change in 
imbibition mechanism caused by small hydrostatic pressure differences across a NAPL 
blob. This is due to either a change in Bond number fi-om density contrast changes. When 
capillary forces dominate, NAPL blobs become isolated fi-om the main body of continuous 



69 



fluid once an imbibition event occurs. Each NAPL blob will have a few to several pore 
openings across which an imbibition capillary pressure is maintained. With an increase in 
the trapping number, specifically the Bond number, the tendency for imbibition to occur 
into the upper (for a DNAPL) region of a vertical pore increases since the hydrostatic 
pressure between the region increases. If this additional hydrostatic pressure is suflBcient 
to allow imbibition into the upper region first, the blob is mobilized. A similar mechanism 
can apply to reduction of DNAPL saturation by increasing viscous forces except that the 
required supplemental pressure at the leading edge of the blob is provided by the viscous 
pressure gradient. 

Movement of a trapped NAPL globule involves drainage at its leading edge and 
imbibition at the rear. Assuming a completely water wetted random sphere pack, the 
pressure drop required for mobilization (APm, [ML'T^]) is given by the difference between 
drainage and imbibition displacement pressures. At 70% water saturation, this difference 
is 2.%olrp (ct, is the interfacial tension [MT'^] and Vp, particle radius [L]) (Morrow and 
Songkran 1981). The value of the supplementary hydrostatic pressure component due to 
buoyancy effects is: 

APs = 0.546a/ rp (4-1) 

Therefore the ratio of APj/APm is 0.2, and thus it is approximately five times more difficult 
to mobilize entrapped fluid than to prevent entrapment (Morrow and Songkran 1981). 

Another main conclusion of Morrow and Songkran (1981) is that the space 
occupied by residual oil saturations after trapping will generally be a subset of the space 
occupied by the residual saturation prior to any flooding and possible mobilization. This is 



■^: ■ '■.*;■■ 



70 

under conditions where capillary forces are dominant. This seems to indicate that 
whatever information on pore size distribution that can be produced from pores filled with 
residual oil, would be indicative of the distribution for the entire media. 

It was also noted that permeabilities (for a given saturation), obtained when 
residuals are reduced by change in entrapment mechanism, do not necessarily correspond 
to those resulting when residual saturations are decreased by mobilization of trapped fluid 
(Morrow et al. 1988). Although this difference could be present during this study, it 
would not be large enough to effect the entire trapping number significantly. , 
Study Objective , • , 

The objective of this study was to conduct two types of soil column experiments. The 
first was to generate mobilization curves similar to Pennell et al. (1996b) using a cosolvent 
mixture typically used in remediation. The second was to generate "entrapment curves" 
for the same media, using similar fluids. Finally, a comparison was then made between the 
desaturation curves for the mobilization and entrapment studies. 

Materials and Methods 

HPLC grade PCE (CAS 127-18-4) was obtained from Fisher Scientific, Fair Lawn 
NJ. The absolute ethanol (>99.5 %; CAS 64-17-5) used in these studies was purchased 
through Spectrum Quality Products, Inc., Gardena CA. Due to large difference in cost 
and small difference in physical properties, reagent alcohol (Fisher Scientific; 90.4 vol. % 
ethanol, 4.6% methanol, 5.0% isopropanol) was also used when absolute ethanol was not 
necessary. This included column final washings. The water used for the cosolvent 
solutions and for soil column flushing was purified through a Nanopure filtration process, 



■■^ "•;■.,:..:, ^ ■ 71 

and brought to an ionic strength of 10"^ M (350 ppm) with calcium chloride, as done in the 
previous chapter (Stumm and Morgan, 1981). ^ 

Stock solutions of ethanol/water mixtures were made in 1 liter quantities using 
standard volumetric glassware. Volume percentages were based on volumes of water and 
ethanol prior to mixing. Although the final total volume is less upon mixing (thus the 
volume percentages are no longer exact), the difference is minimal (1-2%). Furthermore, 
labeling of these solutions by using these pre-mixed volume fractions is for convenience 
only and exact physical parameters used in calculations are determined later. 
Physical Measurements ► 

Density measurements were performed gravimetrically. Two milliliters of solution 
were measured in a gas-tight volumetric syringe and weighed on a precision Mettler 
Balance (± O.OOOlg). A sample's density measurements were repeated no less than three 
times to ensure accuracy and precision of this technique. Viscosities of solutions were 
determined by a Cannon-Fenske Routine Viscometer (Cannon Instrument Company, State 
College PA). A du Nuoy ring tensiometer (Fisher Tensiomat Model 1 1) was used to 
determine the interfacial tension of all samples. The lower limit of this instrument is 
approximately 0. 1 dyne/cm. Laboratory temperature was well controlled and was 23 ± 
O.S^C. 
Sand Column Preparation 

A small-scale glass column (4.8 cm X 15 cm, chromatography column from 
Kontes Corporation) was used for this study. All end materials shipped with the column 
were removed except for the 40 mesh nylon screen. The soil column was incrementally 
packed with well-sorted Number 30-40 sand. This sand size was chosen so that the pore 



72 

size would be approximately equal to the screen mesh size to avoid entrapment of NAPL, 
yet still contain the sand within the column. Vibration of the soil increments was also 
performed to improve packing characteristics. Once the column was packed it was 
weighed with all necessary column parts attached. The soil mass was weighed by 
difference and the internal volume of the column used to calculate the bulk density. Using 
the particle density of silica sand (2.65 g/cm^) and the mass of sand added to the column, 
the volume of sand (Vg) can be calculated. The porosity of the soil column is then easily 

calculated form the total volume of the column as: r| = (l-Vs)A^f Approximately 15 pore 
volumes of de-aired water (via vacuum) were then pumped through the column and the 
pore volume determined. 
PCE Saturation and Generation of Trapping Curves 

Mobilization studies 

' Experiments were conducted, similar to Pennell's ( 1 996b), to develop a trapping 
number curve for PCE and the ethanol cosolvent mixtures. "Pure" PCE (dyed with <5 x 

10"5 M oil-red-o dye, Fisher Scientific, CAS 1320-06-5) was introduced to the column to 
establish residual saturations. This dye concentration range has been shown not to 
significantly affect solubilization and IFT properties (Pennell et al. 1996b; Young 1999). 
The PCE was introduced in a up flow mode to achieve stable displacement of water. 
When PCE appeared at the top of the column, the flow rate was increased 5 fold to 
increase PCE saturation (Dawson and Roberts 1997). The flow was then reversed and 3 
pore volumes of water pumped through in a down flow mode to displace fi"ee product 



73 

PCE, at a flow rate of 5.0 ml/min. The resulting PCE saturation (^o^pce) was determined 
gravimetrically, based on density difference between water and PCE. 

The column was then sequentially flushed with increasing volume fractions of 
cosolvent, without stoppage. Gradually increasing inlet cosolvent fractions avoided front 
instabilities due to the density differences. At the front, due to dilution, PCE may come of 
solution, creating a macroemulsion. This emulsion may eventually resolubilize as it moves 
through the column, exposed to the higher cosolvent fraction, or elute from the column as 
a macroemulsion. This is not desired, as this quantity of PCE is more difficult to quantify. 
Gradient elution was performed to help avoid macroemulsion formation. 

To determine the amount of PCE solubilized compared to the amount mobilized, 
one of the phenomena must be eliminated to quantify both. The trapping number curve 
was first constructed using aqueous streams (water plus cosolvent) pre-equilibrated with 
PCE. This eliminated solubilization and allowed visual determination of mobilization 
(from purely IFT reduction) based on the PCE phase generated from the column. Similar 
experiments were then conducted on the same sand column using unsaturated ethanol 
mixtures (without any PCE added). PCE saturations and trapping numbers were 
determined and results between the two methods compared. 

For each cosolvent fraction, the run was continued for at least one pore volume to 
ensure the resident fluid was characteristic of the injected fluid. The remaining VoSpcE 
was then determined by visual volumetric measurement of mobilized DNAPL. This was 
done for cosolvent volume fractions 20%, 40%, 60%, 80%, 85%, and 90% EtOH/water 
mixtures. For each nm, a series of trapping numbers (Pennell et al. 1996b) was 



74 

determined, using the predicted IFTs from the batch equilibrium experiments. A plot of 
%SpcE versus trapping number, A^j- was then generated. 
Entrapment studies 

To maintain equilibrium between all fluids for the entrapment experiments, 
independent runs using cosolvent and DNAPL phases which had been previously 
contacted and brought to equilibrium was necessary. Three PV's of a reagent alcohol 
mixture were flushed through the column at a flow rate of 25 ml/min. The cosolvent 
phase (with solubilized PCE) from the desired batch solution was then flushed through the 
column in the upflow mode. Only one PV of this fluid was necessary, as the front of this 
displacement was very efiScient and stable, i.e., no fingering occurred. Subsequently, the 
corresponding equilibrated DNAPL phase (mostly dyed PCE) was introduced into the 
column at 5 ml/min in the upflow direction until production of DNAPL appeared in the 
effluent tubing. Then the flowrate was increased to 25 ml/min until a total of one PV was 
used. Finally, the DNAPL was brought to residual saturation with the same pre- 
equilibrated cosolvent phase. All fluids were introduced into the column at a flow rate of 
5 ml/min, unless specifically noted otherwise. 
Porous Medium Parameters 

The intrinsic permeability (k) of the porous media and hence, the eflFective 
permeability (ke = kknv), was determined following PCE addition by use of inlet and 
outlet pressure difference measurements. Resistance due to the column was measured in 
the absence of packing to allow correction for the resistance due to inlets and outlet 
screens, connections and tubing (Morrow et al. 1988). This resistance was subtracted 
from pressure drop measurements over the filled column to determine pressure drops 



75 



across the media only. A differential pressure transducer (Cole-Palmer Instrument 
Company, Niles, Illinois, 0-5 inches H2O differential transducer) was used to monitor this 
pressure difference. 

Relative permeabilities were estimated based on van Genuchten parameters (van 
Genuchten 1980) determined by Tempe cell (Soil Moisture Equipment Co., Santa 
Barbara, CA) measurements (Figure 4-1). These were compared to the data from 
Morrow and Songkran (1981) and found to match closely with actual data measured in 
porous media (glass beads). This data is reproduced in Figure 4-2. 

Thus, the relative permeabilities determined via the van Genuchten parameters 
based upon the Mualem (1976) method were determined to be adequate for the soil 
column. Measurement of the relative permeability with the pressure transducers was 
initially attempted, but fluctuations associated with column resistance effects and 



:(ii 




0.05 



0.1 



15 0.2 

Moisture Content 



0.25 



= 1 1.8 cm 



0.35 



♦ Data 



■Fitted van Genuchten Brooks&Corey 



Figure 4-1 . Moisture release curve for No. 30-40 silica sand used for these studies, 
conducted via Tempe cell, van Genuchten (1980) and Brooks & Corey (1964) fits are 
based on minimizing the sum of squares of the difference between the actual data and the 
fitted line. 



76 



0.95 



0.8 



£ 

> 

S 
e 
at 



0.8 



O.S 



♦ 


"->^ .. 








^ ^V^ R^ = 0.9071 


^-.^ - • 


"^-^ t 




^ 
■^ 




2 4 6 8 10 12 14 


16 


1 • Morrow Data Mualem Fit Regression Line (Morrow and Songkran data) | 





Figure 4-2. Relative permeability to the wetting phase at less than normal nonwetting 
phase residual saturations: Morrow and Songkran (1982) data shown with regression (R^ 
= 0.907) and fit of this study's Tempe cell data based on van Genuchten (1980) 
parameters and the Mualem (1976) method. 



buoyancy effects, due to sometimes large density differences, made these measurements 
erratic. Although this parameter is not directly measured for this study, this should not 
provide significant error, as differences in relative permeability estimates are minor. 

Results and Discussion 



Entrapment in Homogeneous Sand Column 

Curves for the soil column mobilization experiments relating PCE (DNAPL) 
saturation to the total trapping number are shown in Figure 4-3, with Pennell et al. (1996) 
data shown for reference. The data indicate that mobilization of PCE begins at a trapping 



77 



18 



14 



g 12H 

c 
"5 
I 10H 



8- 



U 

a. 



2' 



i ' i 



■ i I Nil 

♦ , • 

I i i i ! ^ ' 



! i ! i 



■ ! 



4, 



i f 

i 1 i 



! ! > I 



I i i 



i--) 



t i \ 



i H ! 



t i 
i I 



1 MM 



j M i 



I i M j i 


i - \ 


j 


AM i j 

! : i 1 


rm 


I'M 11 

i M h 


ilii 

i i i 1 ■ 


1 
; 


1 M Mil 



1.0E-06 



1.0E-05 



1.0E-04 

Trapping Number 



1.0E-03 



APennell (1996) ■gradient2 xgradient4 ♦gradients * Single Runs 



1.0E-02 



Figure 4-3. PCE Desaturation curve - experimental ethanol data only con^ared to those 
ofPenneUetal. (1996). 



number of approximately 2 x 10"^. Three gradient runs are shown in addition to three runs 
that were conducted independently, without any gradient. These were conducted to verify 
that the desaturation curve for PCE residual was not dependent on mode of flushing or 
previous exposure to lower cosolvent volume fraction flushing fluids. As can be seen, the 
trapping relationship is independent of the mode of flushing. 

The DNAPL saturation percentages that resulted from the entrapment experiments 
were plotted against the run's corresponding trapping number and are shown 
in Figure 4-4. The data from the mobilization experiments and Pennell et al. (1996) data 
are shown again for reference. There were two different series of entrapment experiments 



conducted. The first involved all pre-equilibrated fluids, conducted as described 
previously in Materials and Methods. However, in an efifort to determine the possible 
causes of the difference shown in the figure between mobilization and entrapment 
processes, another series of "entrapment studies" was conducted. These were 
accomplished identically to the previous entrapment method, except that the DNAPL 
phase loaded into the column was HPLC grade PCE instead of PCE equilibrated with an 
ethanol cosolvent mixture. This change does not account for much of the difference 
between the two processes, indicating that mobilization is not heavily dependent on mass 
transfer limitations of a slightly partitioning cosolvent, like ethanol. As long as the 
aqueous phase/NAPL interface is amply supplied with components required to keep the 
interfecial tension to it equilibrium value, proper mobilization or entrapment will occur. 
This is obviously more critical during mobilization, as fi-esh NAPL interfaces are 
constantly being met with the flushing cosolvent phase. 

The slight shifl; between the two entrapment runs (all phases equilibrated versus 
only the cosolvent phase equilibrated) can be possibly attributed to slight differences in 
actual interfacial tensions. The use of equilibrated IFT in the trapping number calculation 
is presumably close to the actual IFT in the sand medium. The use of equilibrated IFTs 
for the cosolvent-equilibrated run may underestimate the actual IFT, and therefore 
overestimate the trapping number. This difference is likely small and lead to the small shift 
of the two trapping relationships shown in Figure 4-4. One of the remarkable features of 
these studies is the extreme linearity of the relationship between the DNAPL saturation 
and the trapping number. Table 4-1 shows the results of a linear regression performed 
through both sets of data. 



79 



18-1 


j 


' M in M Mil 


i] 1 1 iiU I 




W' 


i 


1 1 h^ X ' 1 ; X 


■ i 

j 

Xx 






M' 


! 


: ■ ■ r rM - • 


!•: i i 


IX 






Remaining (%) 


i 

A 

i 


'■' y = -4.1 967Ln{x)- 29.214 \ 
R^ = 0.9992 \ 

ill |f* 4 i i i ^ 


■• . X 

Hi . 




! 

! 

i 

• 

i 




1 8 




. I Mil: y = -4.1904Ln(x)- 30.042 \\ ^ 








U, 6 


^ 


R^ = 0.9958 \\ 

i.jlMi i : : M^^ \ 


11, 

,1 1 i 1 ; , 




4- 








» 


: 


! M i i n . ! ! n : li i ! 










.,,.,. ^ 




1.0E-06 


1.0&05 1.0EO4 1.0E-03 1.0EO2 




Trapping Number 


APennell(1996) 


■ gradient2 Xgradient4 •gradients O independent runs - cosolvent equilibrated + all equilibrated fluids | 



Figure 4-4. PCE desaturation curves for both mobilization and entrapment studies, with 
linear regressions shown for the entrapment experiments 



Table 4-1. Results of linear regression of entrapment studies 



Data Series 



Slope y-intercept R_ 



Entrapment -4.1904 

(all pre-equilibrated) 

Entrapment -4.1967 
(PCE not pre-equilibrated) 



-30.042 0.9958 



-29.214 



0.9992 



It is worth repeating that each data point is done independently from the others. 
The slopes of the regression of both data sets are nearly identical. Therefore, the slope 
appears to be independent of phase equilibrium. As the x-axis is representative of the 
capillary pressure through the capillary number, it appears the slope represents a factor 






80 

relating to the pore size distribution of the media. Taber (1969) stated the similarity 
between curves of capillary number and percent saturation and standard capillary pressure 
curves was "obvious". He further stated this similarity should be expected since both 
processes represent the displacement of a fluid from capillaries of various sizes by a 
different and immiscible fluid. Thus, the pore size distribution of the porous mediimi 
should affect both processes in a similar way (Taber 1969). Of all the factors included in 
the Trapping Number, the effect of alcohol addition on trapping and mobilization 
phenomena in these type of studies is due to change in IFT, and not changes in other fluid 
properties (Ryan and Dhir 1996). If this is the case, trapping number curves should 
provide us with similar information as capillary pressiire curves, which are heavily 
dependent on IFT. Separate air-water desaturation studies conducted on the same sand 
using a Tempe cell resulted in a Brooks-Corey lambda of approximately 3.65 (see Figure 
4-5). Previous researchers have stated the space occupied by residual oil saturations will 
generally be a sub-set of the space occupied by the normal residual saturation (Morrow 
and Songkran 1981). This method of obtaining pore-size information has not been found 
to date in previous literature. 
Effect of Pore Size Heterogeneity on the Entrapment of PCE 

Similar to the totally equilibrated entrapment studies discussed above, another 
series of experiments was conducted on the one-dimensional sand column filled with a 
widely graded sand mixture. This sand medium consisted of equal weight fractions of 
#20-30, #30-40, #40-50, #50-60, #70-80, and #80-100 sands. The drainage curve and the 
pore size distribution of this mixture are shown in Appendix A. 



'•H^ -w 



81 



c 
o 



« 0.1 



o 

o 

it 

III 



0.01 



\ 


■ . 

y = 8215.7x""' 
R^ = 0.9989 

\ 


















\ 


■.'■■"' 















10 



100 



Capillary Pressure, cm HjO 



Figure 4-5. Effective saturation of study 30-40 mesh sand as a function of capillary 
pressure, resulting slope of regressed line is the Brooks and Corey lambda. A, = 3.65. 



Two methods of packing the colimin were attempted - wet and dry, both with 
subsequent vibration. The wet packing was accomplished in 3 cm layer with only about 1- 
2 cm of water above to keep it fully saturated. This was done to minimize the distance of 
travel for the different particle sizes with varying settling velocities. However, after 
completion, significant heterogeneity (layering) was observable. This packing was still 
used for study and results are shown below. 

To minimize the layering, a quick fill of the sand mixture under dry conditions was 
also done. Subsequent vibration necessitated the addition of a small layer of new sand at 
the top of the column. The column was then saturated with water fi-om below via vacuum 
aspiration. 



82 



»-. > 



The results of the wide distribution packing are added to the desaturation curves 
presented above and this is shown in Figure 4-6 below. As shown in Figure 4-6, the wide 
distribution and the homogeneous entrapment studies do not behave similarly. It was 
expected that the slope of the entrapment curve for the wider pore distribution would be 
less, resulting in a more gradual desaturation curve. However, it appears that the behavior 
is exactly the opposite. The data reveal that the saturation generally increases with higher 
trapping numbers (lower interfacial tensions). This may be due to PCE being able to enter 
smaller and smaller pores as the interfacial tension between it and the equilibrated 
cosolvent decreases. Additionally, small layered zones of finer media in the column may 
allow fluids with lower IFT to enter and never be able to come out. As previously 



18 



w^ 



14 



S! 



12 



10 



IS 

Ui 6H 
U 



4- 



1.0E-06 



I : 



Xx 




Mii^ 



i ! 



i iil 

M i i 



i I 



1 I i 



! i M ' 
I h 

I h 11. 



1.0E-05 



1.0E-04 

Trapping Number 



1.0E-03 



1.0E-02 



|«gradient2 Xgradient4 •gradients O Single Runs -Entrap PCE +allpfesal ♦ 20-100 1 

Figure 4-6. Results of entrapment experiments on the heterogeneous packing (#20-100 
sand), shown with homogeneous entrapment and mobilization results for reference. 



83 



mentioned, Morrow and Songkran (1981) concluded that it is approximately five times 
more difficult to mobilize than to prevent the entrapment of a NAPL. Therefore, it is 
possible for the DNAPL to enter more pores at lower IFTs and subsequently not be able 
to as easily be mobilized back out. This behavior was not observed in the homogeneous 

>* 

packing since relatively all pore sizes are similar in size. 



Conclusions 

Entrapment and mobilization of residual NAPL are separate and distinct processes. 
This difference can be seen if both processes are plotted on a trapping number curve. The 
entrapment process, represented by the percent of remaining DNAPL saturation (% 
Snapl), appears to be log-linearly related to the trapping number. The exact interpretation 
of this relationship is not clear now, but it is believed to be associated with the log- linear 
dependence of saturation with capillary pressure. This is similar to the Brooks-Corey 
relationship shown in Figure 4-5. 

Dependence of the entrapment process on media heterogeneity is not clearly 
shown. It was expected that the slope of the entrapment curve for the heterogeneous 
media would be less than that of the homogeneous sand, indicating a more gradual release 
ofNAPL throughout the wider range of pore sizes. Difficulty in truly reproducing 
isotropic heterogeneous packing may have contributed to the scatter of data for the vsdde 
pore size distribution packing. However, it is plausible that due to lower permeability 
zones in the packing, the reducing IFT allows additional PCE/DNAPL to remain in these 
smaller pores, increasing saturation. This may be a negative factor in choosing to use 



84 



gradient elution of DNAPLs, as reduced IFTs ahead of any mobilized DNAPL could 
entrap contaminant in smaller pores, leading to longer removal times and possible lower 
removal efficiencies. 



CHAPTER 5 

MOBILIZATION AND ENTRY OF DNAPL POOLS INTO FINER SAND MEDIA: 

TWO-DIMENSIONAL BOX STUDIES 



Introduction 

In-situ flushing remediation is quickly becoming a popular method to remove 
source-zone contamination. Whether using surfactants, alcohols, or oxidants as injection 
fluids to accelerate the displacement, dissolution, or chemical transformation of 
contaminants, control of contaminant movement is critical. Control is critical not only 
during the flushing process to improve recovery and to minimize environmental impact, 
but consideration of contaminant control is vital during the planning and proposal stages 
as well. Proposals to property owners, local, state and federal government agencies are 
more likely to gain approval after sound recommendations and strategies for contaminant 
control have been outlined. The basis for these a priori strategies often include theoretical 
chemical and hydrologic calculations or modeling, but the most valuable input arises from 
field experience. Test cells constructed to study flushing technologies, including one at 
Hill Air Force Base (AFB), Utah (Annable et al. 1996) and one currently being used at 
Dover AFB, Delaware provide excellent opportunities from which to draw conclusions 
and apply them to the "open-field" real remediation situation. However, an important 
experimental method that lies between these two study options in scale, is the use of a 2- 
Dimensional (2-D) box or chamber to study the movement and remediation processes of 
these flushing chemicals. A good review of 2-D laboratory experiments can be found in 

85 



86 

Chevalier and Peterson (1999). 2-D boxes provide not only the horizontal dimension 
tosimulate the hydrologic flushing process involving injection and extraction wells, but the 
added vertical dimension. This vertical dimension becomes important when studying non- 
aqueous phase liquids (NAPLs) that are much lighter or heavier than water or the flushing 
fluid. In the case of dense non-aqueous phase liquids (DNAPLs), movement downward 
and out of the hydrologic control of the remediation flow paths, is undesired. 

NAPL migration in subsurface environments is affected by: (1) volume of NAPL 
released; (2) area of infiltration; (3) time duration of release; (4) properties of NAPL; (5) 
properties of the media; and subsurface flow conditions (Feenstra and Cherry 1988). A 
cross-sectional schematic of the distribution of organic chemicals resulting fi"om a release 
of a DNAPL is depicted in Figure 5-1 . DNAPLs percolate through the unsaturated 
(vadose) zone due to gravity effects leaving behind trapped DNAPL globules and 
volatilized constituents in the gaseous phase. Some lateral spreading occurs due to the 
effect of capillary forces (Schwille 1988) and due to slight media heterogeneity in the 
vertical dimension (layering). Similarly, as enough DNAPL is introduced to the medium it 
can move through saturated zones leaving behind trapped globules (residual saturation). 
This entrapment process is due to interfacial tension effects and thus capillary forces. The 
residual DNAPL can solubilize into water moving through the saturated zone forming a 
contaminant plume downstream, and due to their low water solubility can serve as a long- 
term source. Eventually, large DNAPL volumes migrate down to a zone that has much 
lower permeability than the zone in which it resides. Therefore, it spreads horizontally on 
top of this finer medium until equilibrium conditions are achieved. This resulting zone of 
contamination consists of high saturations of DNAPL (approximately 50% of the pore 



87 



D 



IZ— ICZDD 




Confimiig lavei 



Confined Aquifer (drinking water source) 

Figure 5-1. Schematic of DNAPL contamination of subsurface aquifer systems, showing 
fi-ee phase and residual DNAPL. 



volume), high enough to be considered "pooled" on top of the finer, NAPL capillary 
barrier. 

This process is understood very well conceptually. A good discussion is found in 
McWhorter and Kueper (1996). It is clear that the maximum capillary pressure occurs at 
the base of a DNAPL pool. This pressure is directly proportional to both the pool 
thickness and the density difference of the two fluids. The DNAPL accumulates above the 
finer layer since the capillary pressure due to the pool does not exceed the displacement 



88 

pressure of the "aquitard." Entry of the DNAPL into the less permeable finer layer is 
given by (McWhorter and Kueper 1996), 

l^pgt = Pa (5-1) 

where Ap [ML'^] is the density difference between the DNAPL and the fluid resident in 

the smaller pores below, t is the pool thickness [L], g is acceleration of gravity [LT"^], and 

Pd is the displacement pressure of the finer layer [ML'T^]. Converting pressures to head 

leads to the following equation, 

Apr _j^csidm,pi ' ' ' "^ (5-2) 

~"d 



Pdnapl 

where pdmpi is the density of the DNAPL upon entering the finer layer and hd.cs/dnapi is the 
displacement head of the finer layer when DNAPL is displacing cosolvent filled pores. 
This value, hd,cs/dnapi, , can be determined via. 



/w ycs/dnapl^^^"cs/dnapl _ tcs. (5-3) 



where /j^" is the air-water displacement head and ^ is the contact angle of the fluid pair. 

Note that the ratio of contact angles is approximately unity. This is thought to not 
contribute significantly for these estimations and is therefore excluded. However, as 
complexity is introduced by in-situ flooding chemicals and their associated chemical and 
physical properties, the movement of contaminant becomes more diflBcult to predict. This 
is especially true when dealing with extremely heterogeneous media, or even a simple one- 
layered system. 



89 

Prediction of DNAPL mobilization into and through an underlying finer medium is 
desired before a specific flushing strategy is proposed, or even employed at a remediation 
site. Cosolvents, such as ethanol, are used to increase the rate of dissolution of the 
contaminant pool and associated residual zones into the flushing alcohol mixture. 
However, concurrent interfacial tension reduction can become severe, especially at high 
alcohol volume fi-actions, allowing DNAPL to mobilize out of pores it was previously 
entrapped in and enter smaller pores. If the IFT and buoyancy forces are severe enough, 
this may allow the DNAPL to enter the smaller pores of the underlying "less-permeable" 
layer upon which it originally was pooled. Predictions of the difference in permeability 
(pore size) required to prevent entry into a finer layer, under specific flushing regimes 
would be beneficial. Thus, a systematic approach of determining DNAPL entry into an 
underlying finer layer, using a 2-D box setup with known media sizes, is warranted. Basic 
force balance calculations exist to mathematically predict whether entry into smaller pores 
is possible. Visualization and thus verification of this is not possible in the field, so use of 
2-D setup is justified fiirther. 

Two-dimensional studies of removal of NAPL fi-om porous media have been 
published. Numerous studies exist which focus on the flow instabilities resulting fi-om 
density and viscosity differences, especially in historic petroleum recovery journals 
(Morrow and Songkran 1981). More recently and more applicable to this study, Jawitz et 
al. (1998a) examined the flow instabilities resulting fi-om density and viscosity contrasts 
between resident and displacing cosolvent (ethanol). They concluded that the presence of 
a capillary fiinge and subsequent trapping of cosolvent contributed to the its ineflScient 
removal from the aquifer. However, no NAPL was present to determine possible 



90 

mobilization or impact on flow paths. Other studies on eflFects of heterogeneities and 
instabilities are Kueper and Frind (1988), Held and Illangasekare (1995), and Illangasekare 
et al. (1995). Additionally, studies on dissolution of residual NAPLs have been published, 
including Cellar and Hunt (1993), Miller et al. (1990), and Powers et al. (1994). 
Dissolution of pooled DNAPLs was investigated by Johnson and Pankow (1992). Grubb 
et al. (1996) investigated the removal of a light NAPL (LNAPL), toluene, using a 
combined pure and 50/50 (vol. %) ethanol-water flooding strategy. Downward 
mobilization of the LNAPL below the lighter overriding flushing phase eventually resulted 
in trapped LNAPL. The use of the heavier 50/50 mixture subsequently removed this zone 
via solubilization and physical displacement. Pennell et al. (1996a) qualitatively studied 
the dissolution of PCE and the downward movement of a DNAPL pool in sand and 
aquifer material while flushing with surfactant solutions. They concluded that mobilization 
of DNAPLs via surfactant flooding should be avoided and dissolution of DNAPLs should 
be the primary removal mechanism. 

In summary, little research has been published on a systematic experimental 
approach to predict and verify mathematical relationships describing NAPL (and more 
specifically DNAPL) entry into finer media under cosolvent flooding regimes. This was 
the focus and objective of this research. The methods and results are discussed below. 

Materials and Methods 

A 2-dimensional (2-D) box, previously constructed by Jawitz (1998a), was used 
for this study. The overall dimensions of this box are 61 cm in width, 39.4 cm tall and 1 .4 
cm thick. The inlet and outlet wells were square alimiinum tubes, with 0.05 mm slots 



91 

spaced at 5 mm intervals. The bottom of the box was the same aluminum tubing, without 
any perforations. Together, this aluminum square tubing made up both sides and the 
bottom of the 2-D box. Clear glass, matching the dimensions of the tubing layout, was 
used and was 0.5 cm thick. The 1 .4 cm thickness of the 2-D chamber was over 16 times 
the largest grain size used in these studies. This thickness was chosen by Jawitz et al. to 
minimize wall effects (1998a). Similar to their studies and the studies of Schincariol and 
Schwartz (1990), dye traveling only in the first few grain diameters against the glass 
would appear lighter in dye color than the bulk front. 
General Packing Procedure 

Nanopure water, adjusted to pH 8, was added to the box and the box leak 
checked. The pH adjustment was necessary to minimize adsorption of the Brilliant Blue 
FCF dye (Erioglaucine A, CAS 94082765, Fluka Chemical, Ronkonkoma, New York) to 
sand used in these studies (Jawitz et al. 1998a). Flury and Fluhler (1995) found that as pH 
increases Brilliant Blue FCF dissociates to a mono- and eventually to a bivalent anion 
(pA^ai = 5.83 and pKa2 = 6.58). This pH adjustment ensured that the dye would be in the 
bivalent anionic form, which minimized adsorption to the sand used. Brilliant Blue FCF 
has low adsorption (K4 of 0.19 dm^/kg) in soil with low organic carbon content (0.43%) 
and a soU pH of 5.8 (Flury and Fluhler 1995). Number 20-30 Ottawa Sand (U.S. Silica) 
was used as the constant background media for all 2-D experiments. This media was 
sieved out of the bag and no further treatment was necessary for these investigations. The 
coefiBcient of uniformity is estimated to be 1 .2 and the manufacturer reported the 
roundness and sphericity coefficients of 0.8-0.9 for this sand (Grubb et al. 1996). The 
sand can therefore be classified as rounded-subrounded. This was added to the box in a 



layered fashion (each layer approximately 2 cm thick), with vibration applied at the end of 
each layer addition. Note all packing was done under water wet conditions. The 
subsequent layer was then added and mixed with the upper portions of the previous layer 
to minimize layered effects. This was continued to a depth of 3.3 cm from the bottom of 
the box. Then a 1 cm thick lens of finer media (this media size varied) was added. 
Application of this finer media within 5 cm of either well screen was avoided due to 
possible grain loss through the well screen, especially for the finer media. Upon vibration, 
this settled and spread to a distance of approximately 3 cm from either well. Again, the 
backgroimd No. 20-30 media was added in layers, vibrated, and mixed up to a total depth 
of 1 1 cm. This packing procedure was repeated for each scenario to provide as much 
hydraulic and media consistency possible. This packing method resulted in a pore volume 
of 325-330 ml, and a porosity of 0.35. These figures were constant over all packing 
combinations as the finer layer contributes little to the total 2-D box parameters. The 
following sand sizes were used for the fine media: Nos. 100-140; 60-70; 40-50; and 30- 
40. The particle sizes of the sands used are shown in 

Table 5-1 for reference. A typical box configuration prior to flooding is shown in 
Figure 5-2. 



Table 5-1 . Particle size ranges of sands used. 



Sand Mixture 
(Sieve Numbers) 


Maximum particle 
diameter (mm) 


Minimum particle 
diameter (mm) 


20-30 


0.841 


0.595 


30-40 


0.595 


0.420 


40-50 


0.420 


0.297 


60-70 


0.250 


0.210 


100-140 


0.149 


0.105 



93 



so oec 9ff 
• ^Ac\^GR00MO- 20-'5O 

' rm£(^ L^NEi?" "40-^0 




Figure 5-2. Typical 2-D box setup after injection of PCE, prior to any flushing. 



Dye Tracer Displacement 

To determine the hydrodynamic characteristics of the 2-D flow system, and to 
qualitatively visualize the baseline flow patterns, 30-50 ml of the Brilliant Blue FCF dyed 
water (approximately 50 mg/1) was injected into a colorless, water-resident medium at a 
flowrate of 3.5 ml/min (5.0 ml/min for Rims I and II). This was subsequently flushed 
through the box with colorless water under controlled hydraulic conditions, similar to 
those used during actual flushing runs. This concentration of dye results in a density 
increase of 0.005% (Jawitz et al. 1998a). A flow rate of 3.5 ml/min equates to a 
horizontal flow velocity of 9.7 m/day (13.9 m/day for Runs I and II). For all experiments, 
the profile of the dye fi"ont was traced at generally 5 minute intervals, as the fi"ont moved 
across the flow chamber. As in Jawitz et al. (1998a), the mixing zone at the interface 



94 

between the colorless resident fluid and the dyed displacing fluid was generally less than 1 
cm wide. In situations when the mixing zone had a width of more than 1 cm, the location 
of the front was concluded to be at the center of the visible mbdng zone. This was 
accomplished to determine background flow profiles to eventually compare them to 
profiles with DNAPL pools present. 
DNAPL Introduction 

HPLC grade PCE (CAS 127-18-4), colored red with Oil-red-0 dye (< IxlO^'M, 
CAS 1320-06-5) was injected into the sand media approximately 1 cm above the fine 
layer, using a 1 6 gauge long stainless steel needle, attached to a 20 ml glass syringe. The 
rate of injection varied due to difficulties with PCE traveling back up the needle to the 
sand surface. This was minimized by vibration of the media around the needle after 
insertioa However, DNAPL zone shapes and satiirations were reproduced in a visually 
consistent manner via this method. Generally 2.7 to 3.5 ml of PCE were injected and 
remained in the media. Any PCE on top of the sand media was removed by suctioa 
Hydraulic Controls During 2-D Box Experiments 

The influent was maintained at constant head with a Marriott Bottle, with the head 
adjusted to maintain the water table right at the surface of the sand media. The effluent 
flow was maintained by a Master Flex pump at 3.5-5 cmVmin. The flowrate was 
determined to avoid total well desaturation, depending on the maximum viscosity 
expected from the flushing fluid. The influent line was split by a nylon T- valve to provide 
for easy switching of injection fluids. 

At least one pore volume of background water was passed through the media to 
establish hydraulic equilibrium. 30 to 50 ml of dyed flushing phase was then injected with 

.■■■■'- 1 '■ : 



95 

a Harvard 22 syringe pump at a flow rate equal to the effluent rate. This was done to 
provide visual detection of flushing front and override characteristics. Flow was then 
switched over to non-dyed flushing fluid. The alcohol used as the cosolvent in all flooding 
studies was reagent grade alcohol (Fisher Scientific; 90.4 vol. % ethanol, 4.6% methanol, 
5.0% isopropanol). The reagent alcohol was assumed to have the same properties as pure 
ethanol (Grubb et al. 1996). Isopropanol and methanol should have minor and 
compensating effects on mixtiire equilibria (Sorenson and Arlt 1980). 

Results and Discussion " 

For all floods described below, steady state flow conditions were established prior 
to injection of tracer or alcohol. All experiments were conducted at a room temperature 
of 23 ± rc. Between each run that used the same 2-D packing, at least five pore volumes 
of water was flushed to remove all quantities of alcohol from the sand media. Rim 
summaries are presented in Table 5-2. 
No. 100-140 Fine Layer 

Step input of 100% alcohol ^. ,, .. ' ^ 

The DNAPL volimie was 2.7 ml. The original DNAPL zone shape can be seen in 
Figure 5-3. 100%reagent alcohol was used as the flushing agent. This was done to 
provide a worst-case scenario for this media combination. At roughly one-third of a pore 
volimie, collapsing of the DNAPL pool was noticeable, as IFT's were being reduced and 
mobilization of high saturations was possible. 





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Figure 5-3. Dyed PCE injected into Number 20-30 medium (approximately 2.7 ml) 
pooled over a 1 cm layer of Number 100-140 medium. 



Solubilization diminished the size of this zone and light red 'streams' developed 
downstream. This reddish color is due to slight partitioning of the Oil-red-0 dye into the 
flushing phase. Partitioning of the dye becomes increasingly possible due to the high 
amounts of PCE solubilized into the alcohol (>200,000 mg/1). This "banding of dye" has 
been seen several times in one-dimensional columns and generally occurs when 80 to 85% 
alcohol is used as the flushing phase. Due to dilution, this rough concentration can occur 
ahead of the 100% alcohol front. The equilibrated volume fractions resulting from this 
mixture from bulk studies is approximately 60% alcohol/28% PCE/ 12 % water. 

By 40 minutes into the flush (0.62 PV) fiirther collapse of the pool down onto the 
finer 100-140 layer resulted in a layer of DNAPL ranging from 0.4 to 0.5 cm in thickness. 
This pool was most prevalent ahead of the original pool area and spread downstream as a 
fimction of time. As the PCE-alcohol IFT was decreasing, DNAPL from upstream 



98 

portions of the original pool (above residual saturations) mobilized quickly in the coarse 
medium, in a direction along the alcohol front and eventually into the finer layer. This 
phenomenon will be hereafter referred to as "frontal mobilization". This was noticed as 
early as 3 1 minutes into the run (~ 0.5 PV). This mobilization occurred in a very thin 
stream, most likely due to the very sharp interface between the displacing alcohol and the 
resident water, which is on the order of tenths of centimeters (Grubb et al. 1996). Similar 
"frontal mobilization" observations have been made by others (Grubb et al. 1996; Pennell 
et al. 1996b), where DNAPL flows dovraward due to remaining higher density, yet seeks 
pores of reduced IFT and thus can flow back against the hydraulic gradient. Further 
breakthrough of PCE into the finer layer occurred 36-38 cm from the injection well at 
t=50 minutes (0.77 PV). This occurred when the alcohol front had sufficiently passed into 

-r 

the finer layer underneath, allowing mobilization into the finer pores. 
One-dimensional horizontal sand column experiments 

To better understand flow behavior of residual DNAPLs under the presence of 
alcohol containing cosolvents, 1-D sand columns were brought to residual saturation vsdth 
dyed PCE. The procedures and setup for this are explained in Chapter 3. The column 
was turned horizontal and alcohol injected from left to right (see Figure 5-4). Gradient 
ethanol injections of to 100 % v/v ethanol over one pore volume were used in an 
attempt to minimize override. As can be seen in Figure 5-4, this had little impact and 
override of the cosolvent still occurred. This is due to not only density difference, but also 
the eventual contrast in relative permeability caused by the downward moving DNAPL. 
High saturations of PCE developed near the cosolvent interfece and globules could be 






99 

seen to move diagonally downward along the interface. Eventual pooling developed along 
the bottom and near the inlet end of the column (Figure 5-4). 

This appears to support what occurs when DNAPL saturations are above residual 
quantities, as the case in the 2-D box experiments. The higher saturation or pooled 
scenario would be expected to behave similarly, if not in a more dramatic fashion. 




Figure 5-4. Removal of residual dyed PCE by gradient ethanol injection (0-100% v/v) 
over one pore volume. Darker band at interface is highly saturated PCE which is 
mobilizing toward the lower left and pooling. ' ?^ " . 



Step input of 80% alcohol 

3.2nilofPCE was injected as described in procedures above. No mobilization of 
DNAPL was observed during this entire run. Very clear progression of pool collapse 
occurred as shown in 

Figure 5-5, eventually resulting in an extended pool thickness of 0.2 



100 




Figure 5-5. Progression of DNAPL pool collapse - Nos. 20-30 background medium, 
Nos. 100-140 finer layer - after 0.8 PV of 80% v/v ethanol/water step input. Downstream 
direction is to the right in all pictures. 




Figure 5-6. Spreading of DNAPL pool downstream on top of finer Nos 100-140 layer. 
No breakthrough occurred during this run - 1 .1 PV after 80% v/v ethanol/water step 
input. 



■''■ \'i ■■■ 101 

to 0.4 mm. The spreading of the pool occurred only downstream of the injection zone ( 
Figure 5-6). No upstream spreading of the DNAPL was observed. 
No. 60-70 Fine Layer 

Step input of 80% alcohol 

Based on the lack of mobilization into the 100-140 layer using an 80% alcohol step 
input, the next scenario chosen was to flood using the same flushing fluid, but decrease the 
contrast between the bulk and finer layers fi*om Nos. 20-30 vs. 100-140 to Nos. 20-30 vs. 
60-70 mixture. 4.5 ml of PCE was injected for this run. Collapse of the pool occurred 
with upstream mobilization on top of the finer 60-70 layer. The pool spread 
approximately 6 cm toward the injection well and 25 cm downstream fi"om the injection 
zone, eventually draining off the edge and flowing vertically downward due to density 
differences. This occurred only in this scenario; most likely due to the increased amoimt 
of PCE injected (4.5 ml). However, no entry of fi-ee phase DNAPL was observed into the 
finer layer. See schematic ofrun in Figure 5-7. 
Gradient Iniection (10-90%) of Alcohol 

Initial 2-D box trials were conducted using a similar setup to determine the overall 
benefits of gradient injection of alcohol over step input to 100% alcohol. However, no 
finer layer was present, in the bulk 20-30 sand medium. Increased mobilization of 
DNAPL was observed during the gradient injections than with the step input to 1 00% 
alcohol. A conclusion made was that interfacial tension was quickly decreasing during 
gradient injection, yet the cosolvent's ability to solubilize PCE was not keeping pace 



102 



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103 

with the falling IFT. In the case of a step input, although IFTs are very rapidly reduced, 
the capacity of the pure alcohol solution to solubilize PCE overwhelms the IFT reduction. 
Essentially the saturation of DNAPL reduces faster due to solubilization, quick enough to 
prevent mobilization. This preliminary conclusion was tested again with the 2-D box 
setup described here. 

A DNAPL volume of 3.1 ml was injected into the 20-30 medium, resting on a 60- 
70 medium. A gradient injection from 10% -90% (v/v) alcohol was applied over 1 PV 
into the 2-D box, using a Shimadzu HPLC pump with a solvent mixer. At 58% alcohol 
the alcohol influent line was switched over to a Brilliant Blue dyed 100% alcohol reservoir 
and not removed until 75% alcohol was injected into the 2-D box. The alcohol influent 
line was then returned to the clear 100% alcohol resulting in a blue band of alcohol phase 
representing a concentration range from 58 to 75% alcohol. Typical collapsing of the 
DNAPL zone was observed, with the most significant movement occurring under an 
alcohol concentration of approximately 50% by volimie. Again, horizontal spreading was 
observed upstream, as well as downstream, from the injection zone as shown in Figure 
5-8. Light red bands of PCE-containing alcohol entered into the finer media, moved 
through the layer and then exited into the coarser media below, where due to density 
override, lower alcohol concentrations are present (Figure 5-9). This results in PCE 
coming out of the flushing phase and reestablishment of a separate DNAPL phase (see 
Figure 5-10). This supports remediation designs that completely flood underneath the 
contaminant zone and supporting finer layer with 100% alcohol. Pre- established presence 
of pure alcohol would severely minimize this regeneration of a new DNAPL phase. 



104 




Figure 5-8. Horizontal spreading of PCE pool upstream from injection zone - Nos. 20-30 
background media, 60-70 finer layer, 1 . 1 PV after gradient injection of 10 - 90% v/v 
ethanol/water over IPV. Blue band is location of 58% (leading edge) to 76% ethanol 




Figure 5-9. Highly concentrated cosolvent phase in which dye has partitioned, entering 
iiner Nos. 60-70 layer. This is not free phase mobilization. Blue band above is from a 
post gradient step input to 100% reagent alcohol 



105 



f 


H 


H 









Figure 5-10 - Breakthrough of highly concentrated PCE containing cosolvent phase into 
finer layer and subsequent reestablishment of DNAPL below the finer layer due to lower 
alcohol concentrations. 



No. 40-50 Fine Layer . ^■- ' 

Background dye flush after DNAPL injection 

A total of 3.3 ml of PCE was injected for this run. Significant flow of flushing 
phase was observed underneath the DNAPL pool, which was not observed in previous 
runs. The permeability of the 40-50 layer was greater than the relative permeability of the 
DNAPL saturated zone. This caused significant flow of dye through the finer layer, 
underneath the DNAPL pool, instead of override as observed in previous scenarios, with 
finer, less permeable media (see Figure 5-11). Fronts in the finer layer lagged the fi-ont in 
the 20-30 media only by 10 minutes (0.1 PV). 



106 




Figure 5-11. Water tracer study for 40-50 finer layer experiment. Note the significant 
holdup of tracer in lower portions of PCE pool and noticeable progression of dye in finer 
layer underneath. 



Step input of 80% alcohol 

Collapsing of the DNAPL pool occurred causing the PCE pool to spread 
approximately 4.2 cm upstream. The pool spread a total 17.5 cm downstream (see Figure 
5-12). No mobilization of a separate phase occurred into the finer layer, although 
solubilized PCE does breakthrough into the finer layer and into the coarse layer below. 
However, no fi'ee phase PCE was generated below the finer layer. 



107 




so oec 
^IM£R LVSEf?- *^-SO 



Figure 5-12. Collapsing of PCE pool and spreading of DNAPL along 40-50 layer. No 
breakthrough of DNAPL observed. 



No. 30-40 Fine Layer 

Step input of 80% alcohol 

Mobilization occurred in three different locations. Two upstream (1 .8 cm and 6.5 
cm) from the injection zone and one downstream (6.4 cm) (see schematic in Figure 5-13). 
This movement through the finer layer, based on visual observations, was completely 
different from the solubilized movement shown above in Figure 5-9. True DNAPL 
mobilization occurs through one or two pores and networks downward due to gravity 
differences (see Figure 5-15). ' 



108 







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Figure 5-15. Mobilization of DNAPL into finer 30-40 layer at two locations, upstream 
fi-om injection zone (small + mark in picture) - 0.45 PV after step input of 80% v/v 
ethanol/water mixture. Mobilization also occurred later at one other location downstream 
of pool (see text). 



Step input of 70% alcohol 

Similar to the 80% alcohol run, mobilization occurred in three different locations. 
However, only one was upstream (3.3 cm) fi"om the injection zone and two downstream 
(7.4 cm and 10.9 cm). DNAPL globule mobilization was very similar to that of Run 7. 
One area of breakthrough into the finer layer match exactly with one fi"om the previous 
run, indicating the possibility of one preferential channel at that location for this packing 
(see Figure 5-14). 



no 

Stq) input of 50% alcohol 

As the injection alcohol concentration decreases, the solubilizing capacity 
decreases, and longer times are required to remove the DNAPL. However, initial 
upstream mobilization of DNAPL in the coarser layer, from the upstream side of the 
injected pool, occurred approximately at the same run times, during both the 80% or 50% 
alcohol step input. This indicates that the impact of IFT reduction is manifested earlier 
than the reduction in saturation due to solubilization. However, once immobilized on top 
of the finer layer, the IFT reduction is not severe enough within the smaller pores, and 
thus the capillary pressure within these pores is too great for the DNAPL to enter. Similar 
to previous runs, the lateral spreading of the PCE resulted in a DNAPL layer of 0.4 to 0.5 
cm in thickness. After 1 PV of 50% alcohol the flushing concentration was step increased 
to 60% and flushed for another pore volume. This flushing was continued until this 
injected fluid saturated the finer layer and thus a similar prediction if mobilization occurred 
or not made. Mobilization did not occur even for this mixture. This additional pore 
volume of 60% alcohol reduced DNAPL saturation further due to slow solubilization. 
Another step increase to 80% alcohol was made to determine if this reduced saturation 
and thickness of DNAPL on top of the finer layer was still able to mobilize with the new 
80% alcohol flushing mixture. No mobilization was seen during this step increase as well. 
Two-Dimensional Studies with t-Butyl Alcohol 

A swelling cosolvent, t-butyl alcohol (TBA), was chosen to evaluate its effects on 
DNAPL solubilization, mobilization, and breakthrough behavior in the 2-D environment 
studied above. Experimental setup, flow rates and head gradients were kept similar to 
those described above for the reagent alcohol studies. 



HI 

Step input of 30% TBA: #30-40 finer layer 

Dyed PCE (3.3 ml) was injected on top of the finer #30-40 layer. A step input of 
30% v/v of TBA was applied to the box, removed at a flow rate of 3.5 ml/min, with the 
influent head maintained at the top surface of the sand medium. Entry of DNAPL into the 
finer layer occurred at 44 minutes (0.47 PV) as three fingers slowly progressed into the 
finer layer. Progression of DNAPL through the finer layer was noticeably different than 
that observed in the reagent alcohol experiments. Movement of the DNAPL fingers was 
slower in the downward direction and more lateral spreading occurred, most likely due to 
the lower density of the DNAPL (approximate equilibrated density of 1.53 g/ml) caused 
by the partitioning of the TBA cosolvent into the PCE phase. Similar increased lateral 
spreading was observed with DNAPL mobilization in the background 20-30 layer, both 
behind (upstream) and in fi-ont (downstream) of the original PCE pool. 

The volume of the PCE/DNAPL pool did appear to attain a larger volume than in 
previous reagent alcohol runs. Even at larger runs times when significant solubilization 
has occurred, the pool volume remained larger than expected or observed in the reagent 
alcohol experiments. This is again due to the swelling of the PCE fi-om the partitioning of 
the TBA (approximately 15% based on equilibrium studies). 

At 215 minutes (2.3 PV) there still remained a noticeable pink aqueous phase in 
the upper right portion of the finer layer, indicating not all of the solubilized PCE had been 
removed. 
Step input of 40% TBA: #100-140 finer layer 

The 2-D box was repacked with #100-140 sand chosen as the finer layer. This 
was done to determine swelling effects of a higher percentage of TBA while maintaining 



-. i.r? 



112 

all of the DNAPL above the finer layer. This scenario would more closely represent those 
experienced in horizontal flooding field situations, where even a more impermeable clay 
layer is supporting a pooled DNAPL. 

Some desaturation due to air bubbles occurred during packing, especially just 
above the finer layer. Water tracer runs were done (before and after the injection of PCE) 
to quantify the effects of this desaturation (see traces in Appendix B). The effects were 
significant enough to warrant overnight flooding of the box with de-aired water 
(approximately 7 PVs). This removed all visible desaturation except near the extreme 
downstream portions of the finer layer. This was not seen to cause significant problems to 
the fiirther use of this packing, since the key observations are observed far upstream fi-om 
this area. 

A volume of 3 .2 ml of dyed PCE was injected onto the # 1 00- 1 40 finer layer. The 
step input of 40% TBA was applied as previously described. Significant differences were 
observed in pool properties, even compared to the 30% TBA run. No entry of DNAPL 
into the finer layer was observed during this run. All DNAPL remained above the finer 
layer as desired. A definite downstream movement of DNAPL was observed, different 
fi*om that previously observed for the reagent alcohol runs. This movement of DNAPL 
had more of a horizontal characteristic, due to a lower density contrast, compared to the 
reagent alcohol studies. The pool appeared to mobilize with large horizontal components 
until the density contrast became great enough for it to begin to move downward. This 
was apparently due to there initially being more water diluted pores below the DNAPL, 
then as the alcohol fi-ont entered the pores below the DNAPL at a later time (due to 
gravity override), the density contrast became great enough for more downward 



113 



mobilization. In the process of this "two-step" mobilization of DNAPL, quantities of 
injected cosolvent became trapped and isolated on top of the finer layer (see Figure 5-16). 
These pores required significant time to be flushed, due to the relative permeability to the 
wetting phase being so low around these areas. This can possibly lead to long tailing of 
PCE concentrations during the removal process. Periodic samples of the extraction well 
effluent were taken and stored for later analysis by gas chromatography. The results of 
this analysis were used to construct a breakthrough curve for PCE. This is shown in 
Figure 5-17. 




Figure 5-16. Mobilization of the PCE pool by a 40% v/v TBA cosolvent mixture (0.6 PV) 
resulting in the trapping of a volume of the cosolvent mixture on top of the finer layer. 



114 



Pore Volumes 

1.5 2 2.5 



25000 




1.60E+07 



1.40E+07 



1.20E-K)7 



l.OOE+O? T 



8.00E-K)6 ^ 

a, 

6.00E+06 < 

CO 



■ ■ 4.00E+06 



2.00E+fl6 



O.OOE+00 



150 175 200 
Time (minutes) 



Figure 5-17. PCE and TBA elution profiles fi-om 2D Box after a step input of 40% v/v 
TBA/H2O. TBA profile data are shown as GC peak areas for reference. 



Approximately 95% of the PCE was removed after just over 2.5 pore volumes of 
flushing. The linearly decreasing profile fi-om 1 .5 to 2 pore volumes is the result of 
gradual removal (via solubilization) of the swollen pool that has ateady collapsed and 
spread on the finer layer. Sequential removal of the upstream portions of the pool was 
observed. This removal process decreased the surface area of the entire pool available for 
mass transfer and could explain this gradual, yet consistent profile decrease over this 
period. PCE concentrations then leveled off for a short time period at about 2.0 pore 
volumes. This can be due to portions of trapped cosolvent (with high concentrations of 
solubilized PCE) finally becoming available for removal. 



115 

The swelling of the DNAPL pool was definitely observable as shown in Figure 
5-18 and when compared to figures above for reagent alcohol experiments. Batch studies 
with 10 ml of 40% TBA/H2O and 10 ml of PCE resulted in an equilibrated DNAPL 
volume of 13.5 ml, indicating a swelling of approximately 35%. Thus, the 3.2 ml 
originally injected could potentially swell to a volume of 4.3 ml. The volume shown in 
Figure 5-18 is difficult to estimate due to the variability of DNAPL saturation. However, 
a rough estimation of the entire bulk volume of the pool is 24 cm^ which assuming a 
porosity of 0.35, leads to a pore volume of 8.4 cm\ Assuming 50% DNAPL saturation 
would result in a final estimated DNAPL volume of 4.2 ml. ; 

•-■ — .-v^^- _ ■■,- . .■••.:,;.K;:,.-. •■•■" ' '''':>^~^'^^^W>'^Sf^'^^rf~tf:?)^'^!'-'-}rr^ 




iOl % T%A -TEC ifjPuT 



i^iMii 



1Q^^^.V^SS^<K^1^<: 










Figure 5-18. DNAPL pool shape after the injection of one pore volume of 40% v/v TBA 
cosolvent mixture. 



116 

The "two-step" mobilization process discussed above appeared to continue as the 
pool progressed downstream. As a result, a discontinuous thinner layer (0.4-0.7 cm thick) 
of DNAPL developed. This sub-layer of DNAPL was considered to be in direct contact 
with the pores of the finer layer, and used in the subsequent entry pressure calculations. 
Systematic Quantitative Evaluation and Prediction of Mobilization into Finer Layers 
To explain the above results and determine if their qualitative nature matched what can be 
estimated based on porous media physics and the hydrology of each scenario, calculations 
based on air entry pressures for each media used were accomplished. Air entry pressures 
were determined using Tempe Cells (Soil Moisture Equipment Corp., Santa Barbara, 
California). Total desaturation profiles were produced as a result of these measurements 
(Appendix A). The desaturation data were fitted v^th both Brooks and Corey (1964) 
parameters and those developed by van Genuchten (1980). Curve fitting was 
accomplished by minimization of the sum of squares of the differences between the data 
and the fitted prediction. Spreadsheet solver macros were use to iterate and arrive at a 
minimized error. The parameters resulting fi"om these curve fits are presented in Table 
5-3. DNAPL entry values were then calculated for each scenario based on the ratio of 
IFT of the fluids and the IFT between air and water measured with a du Nuoy ring 
tensiometer (72.1 dynes/cm). This then incorporates pore size into the calculation based 
on this entry value. The force balance associated with the entry pressure was presented in 
the introduction of this chapter. Entry of a DNAPL globule into the finer media can only 
occur if the head caused by the height of the globule can overcome the capillary head of 



117 



s 

o 



> 

B 
o 



d 

U 
1 

u 

o 



<L> 



<U 



-*« 
V 

s 

2 

a 
g 


Average pore 
radius, mm 


o 


d 


o 
d 


o 
d 


(N 

o 
d 


ON 

o 
o 

o 

d 


S 


1-H 

o 


o 

On 

d 


ON 

d 


On 

d 


ON 

d 


d 


s 
a 
« 

a 

> 


a 


o 


d 


o 


CN 






8 "a 


o 


o 
d 


o 
d 


O 

d 


On 
O 

d 


(N 

d 


-^^ 

s 

2 

a 

I- 

o 
U 

i 

e 

2 

n 


^ a 

Q. a 

^2 


O 


00 

o 
d 


o 
d 


On 
O 

d 


O 
O 

d 




o 

^ a 

.a 


q 


d 




On 
(N 


d 




.^ 








CO 

On 


On 

ON 
ON 




Sand 

Mixture 

(Sieve No.) 


o 
m 

1 
o 

CM 


O 

o 
m 


O 
o 


O 
O 


O 

o 


o 
It 



118 

the pore below it in the finer layer. Note that densities and IFTs used in these calculations 
were determined by batch experiments reported previously in Chapter 2. This therefore 
assumes that these fluids have reached equilibrated values in the 2-D box at the time of 
possible mobilization Although these parameters may not be exactly the actual values, 
differences would be slight and not significant to the decimal place of these predictions. 
For comparative purposes, the results of these calculations are shown in Table 5-4. 

Except for runs V and X, all calculations accurately predict whether mobilization 
into the finer layer occurred. However, runs V and X cosolvent/DNAPL entry pressures 
into the finer medium (/i^^ "''"'" = 0.38 cm and h'J"''^'" = 0.91cm) are weU within 

reasonable errors associated with estimating DNAPL depth (h''"''^') alone. No conclusion, 
one way or the other, can be convincingly drawn fi-om these runs' estimates. The 
permeability estimates were made by averaging the results of three repetitions of the 
falling head technique through a 1 -D sand column. Column, tubing, and valve resistances 
were separated fi-om the media resistance by conducting "blank" falling head tests on the 
apparatus alone. Table 5-4 results indicate that breakthrough of PCE/DNAPL is not a 
sole function of permeability of the finer layer below the DNAPL pool. It is also a 
fiinction of the cosolvent and the properties of the resulting solution that resides in the 
pores into which the DNAPL can enter, confirming the relationship presented in 
McWhorter and Kueper (1996). Thus, a more accurate parameter to use to predict PCE 
entry into lower layers is h^ '''""''' , the entry pressure of the media by a DNAPL replacing 
an equilibrated cosolvent mixture. This pressure is that required to allow DNAPL to enter 
the equilibrated cosolvent resident finer pores below. From the results presented, a 



O 
0^ 



X) 

o 



o 

•i 
u 

U 



C 

o 

■§ 
I 

O 
u 



(L> ,0 



I I 

A O 

•B 00 

Jt) Si 



.22 > 

DO «iS 



2 '^ 

Z o 

."*"* & 

'S 3 

OQ D- 

o .> 

2 

I .2 

CO M 



^ 



§ o 



<2 



B 

o 
3 "^ 



(U 



X 

o 

CI. 

& 

<L> 






"do 



o 

■^^ 
CO 

I 

_o 

X) 

CO 



O CO 

n CO 

|J 

k. > 

CO JS" 

CO ti 

S & 

_ o 






Did Entry 
Occur? 


>H 


Z 


Z 


z 


z 


>- 


>^ 


z 


Z 


>^ 


z 


■ 


Permeability 

of fine layer 

k 

(cm') 




W 

CO 


00 


00 

r4 




o 


1 

W 
o 
q 

in 


1 
W 
o 
<^ 


r- 

1 

W 
o 
q 

in 


1 
W 
o 
q 


1 
PQ 


w 

m 

o^ 


S" If « 


c5 


d 


d 


d 


d 


d 


d 


d 


d 


00 

d 

1 

d 


q 

in 

d 


1 


5 5 = 


O 


d 


d 


1 


oo 
d 


d 


CO 

d 


ON 

d 


NO 

d 


d 


ON 

d 


>n 


2s? 5^ 






00 


00 


00 


NO 

d 


NO 

d 


NO 

d 

r— < 


NO 

d 


NO 

d 




o 
o 
in 


Ap 
(g/ml) 


o 


d 


d 


1 


d 


d 


00 

d 


OO 

d 


00 

NO 

d 


in 

d 


00 

d 


m 

d 


Pes 

(g/ml) 


o 


o 


On 

d 


d 


ON 

d 


OS 

d 


ON 

d 


On 

d 


ON 

d 


ON 

On 

d 


oo 

ON 

d 


- 


if 

a M 




IT) 


oo 
in 


00 

in 


00 


00 


On 
in 


NO 


NO 


in 


NO 


in 


Ycs/dnapl 

dynes/cm 


w-1 


rN 


CN 


(L) 

> 


<N 


H 




<n 


CO 

NO 


in 


ON 
CO 


in 


%v/v 
Alcohol 


§ 


o 
00 


o 
oo 


o 
o 


O 
00 


o 

00 


O 


o 

NO 


o 
in 


< 
o 


< 

PQ 
H 
o 


o 
o 


Flushing 
Mode 


on 


C/5 


C/3 


O 






Ci. 

<u 

V3 


-4— • 

00 


C/5 


-4—* 

V3 


•4— > 




Fine 

Layer 

(Sieve #) 


O 


O 

o 

o 


O 

1 
o 


o 
o 


o 

1 
o 


o 
-^ 

o 


O 

6 


o 


o 

t 

o 


o 


O 




Run 
Number 


»— 4 






> 


> 


> 




X) 

1—1 
> 


CO 
> 


X 


X 


1 



i ' 120 

guideline to predict entry for a PCE/ethanol system in homogeneous sands is h'J'^^' < 

0.35 cm. For reference, data taken from van Genuchten (1980) for a Beit Netofe clay is 
used to calculate a corresponding entry pressure imder typical cosolvent flushing 
conditions to remove a DNAPL like PCE. Based on these assumed conditions (see Table 
5-4), approximately a half a meter of DNAPL would be required to enter Beit Netofa clay. 
This value seems reasonable. r 

Conclusions 

Removal of pooled DNAPL (PCE) on top of finer, less permeable layers during 2- 
dimensional box cosolvent floods, presents interesting qualitative conclusions. These can 
be supported semi-quantitatively with pore force-balance calculations. 

Pooled DNAPL will collapse under reducing IFT conditions, and if residuals are 
high enough, can mobilize downward and upstream along overriding cosolvent fronts. 
This can cause significant build up of DNAPL on the lower confining layer, upstream from 
a pooled DNAPL system. This allows increased exposure to lower IFT cosolvent 
solutions that may permeate into the finer layer. Not enough time has passed in the flood 
to solubilize significant residuals, therefore allowing increased DNAPL heads to exceed 
entry values of the media below. Downstream mobilization into finer layers can occur as 
well, only after time has elapsed to allow cosolvent to migrate into the pores of the finer 
layer. In general, the most significant production of DNAPL through any fine layer in 
these studies was actually upstream from the source zone. 

Gradient injection to remove DNAPLs does not appear to provide significant 
benefit over step inputs. Override characteristics are not improved upon dramatically. 



. " 121 

Furthermore, interfacial tension between injected fluid and DNAPL decreases almost 
instantaneously compared to removal of DNAPL due to solubilization. Movement of high 
residuals of DNAPL down onto the finer layer occurs well before significant reduction of 
saturations due to solubilization. Here, solubilization of DNAPL becomes even less 
efficient due to essentially only one interface fi-om which to allow mass transfer. 

Entry calculations using the physical and hydrogeologic parameters of the chemical 
phases and media predicted breakthrough of PCE into the finer media in excellent fashion. 
Breakthrough of PCE under typical ethanol flooding conditions (80% v/v ethanol/water) 
can generally be assumed to occur in homogeneous sands when /j^'"*^' < 0.35 cm. Of 

course, several variables, especially amount of pooled DNAPL present, does not allow an 
exact prediction for all instJinces, but a rough prediction based on flooding conditions and 
media contrasts can be made. It is estimated that for a Beit Netofa clay, approximately 
one-half a meter of a PCE-like DNAPL would be necessary to enter imder extreme 
cosolvent flooding conditions (IFT - 0.5 dynes/cm; Ap = 0.5 g/cm^ an air entry pressure 
of 75 meters.) 



» \ 



CHAPTER 6 
SUMMARY AND CONCLUSIONS 



Batch equilibrium studies were conducted to determine adequate methods to 
predict physical properties of PCE/cosolvent systems. This included PCE solubility and 
resuhing fluid interfacial tension. Batch studies resulted in cosolvency powers (ct) of 3.73 
and 4.13 for ethanol and isopropanol, respectively. Use of the log-linear solubility 
relationships appears to be not a completely accurate method to predict solubility of PCE 
in cosolvent mixtures, especially over the entire range of possible volume fractions. The 
log-linear predictions perform best at higher cosolvent volume fractions. Therefore, these 
predictions may be adequate for estimations necessary for field studies or remediation 
efforts. For improved estimation of PCE solubilities over a wider range of cosolvent 
volume fractions, the use of the Extended Hildebrand or UNIFAC models is 
recommended. The added complexity of these models is beneficial for accurate solubility 
predictions over the entire range of cosolvent fractions. 

Additionally, the interfacial tension resulting from various cosolvent mixtures and 
its prediction based on the initial volume fraction of cosolvent leads to an interesting 
relationship that is similar to the log-linear model. An "IFT reduction power" was 
determined for ethanol, QEtoH= -3.60, and isopropyl alcohol, Qipa= -5.80, describing the 
ability of cosolvents to reduce IFT with increasing volume fraction. This parameter 
quantitatively describes the ability of the cosolvent to reduce the IFT as it is added in 

122 



■ - - ;/•" '.■■ ■■■. 123 

increasing volume fractions. IFT can also be accurately estimated by PCE aqueous phase 
solubility, especially in regimes conducive to cosolvent flushing. Due to the dependency 
of PCE aqueous phase solubility upon the aqueous and DNAPL phase ratio and 
partitioning of cosolvent into the DNAPL phase, it should be noted that this approach is 
problem specific. Incorporating this property information into a trapping number 
relationship (PenneU, 1996) allows for improved prediction of remediation schemes 
remaining within solubilization boundaries, avoiding or minimizing mobilization of the 
NAPL/DNAPL phase. 

The trapping number (N,) is an effective parameter to help predict mobilization of 
non-aqueous phase liquids in subsurface environments. Onset of residual PCE mobilization 
was foimd to begin at a trapping number (M) of 2 x 10"^. Trapping number results and 
onset of PCE mobilization were found similar, although slightly greater, to those of 
Pennell et al.( 1996b). Ethanol used as a cosolvent (at volume fractions less than 85%) 
enhanced solubilization of PCE to the point where this process is dominant and 
mobilization of PCE can be avoided in homogeneous media similar to #30-40 U.S. silica 
sand. However, under severe conditions, mobilization using cosolvents can occur. This 
includes large step inputs to high cosolvent fractions, where DNAPL saturation is still 
great eiK)ugh for immediate IFT reduction to cause mobiUzation, at least in a local sense. 
This of course could be inqjortant if, within that locality, DNAPL moves out of the zone 
of control. These issues were further addressed in the two-dimensional box studies. . 

As should be expected, differences between surfectant and cosolvent systems are 
not apparent on a mobilization curve. Furthermore, mobilization curves appear 
independent of alcohol type. However, as partitioning of an alcohol, like t-butanol, into 



124 

the NAPL occurs, the volume of mobilized NAPL and residual globules of NAPL left 
behind increase due to swelling and remaining saturation becomes more difficult to 
quantify. Further research into this area is warranted. 

Entrapment and mobilization of residual NAPL are separate and distinct processes. 
This difference can be seen if both processes are plotted on the same trapping number 
curve. The entrapment process, represented by the percent of remaining DNAPL 
saturation (% Snapl), appears to be log-linearly related to the trapping number. The exact 
interpretation of this relationship is not clear presently, but it is believed to be associated 
with the log-linear dependence of saturation with capillary pressure. However, for 
heterogeneous media a general trend of increased saturations with decreasing IFTs was 
observed. This is thought to be caused by the lower IFTs allowing DNAPL access to 
smaller pores and subsequently not being removed due the increased diflSculty of 
mobilization over entrapment (Morrow et al. 1988). Additional study to confirm this 
phenomenon and its exact justification is needed. 

Removal of pooled DNAPL (PCE) on top of finer, less permeable layers during 
2-D box cosolvent floods, presents interesting qualitative conclusions. These can be 
supported semi-quantitatively with pore force-balance calculations. Pooled DNAPL will 
collapse under reducing IFT conditions, and if residuals are high enough, can mobilize 
downward and upstream along overriding cosolvent fi'onts. This can cause significant 
build up of DNAPL on the lower confining layer, upstream fi-om a pooled DNAPL 
system. This allows increased exposure to lower IFT cosolvent solutions that may 
permeate into the finer layer. If little time has passed since the start of a flood for the 
flushing phase to solubilize significant residuals, discontinuous globules can pool together 



125 

and accumulate on top of the finer layer, resulting in increased DNAPL heads. These 
DNAPL pressures may exceed entry values of the media below. Downstream, 
mobilization into finer layers can occur as well, only after significant time has elapsed to 
allow the cosolvent to migrate into the pores of the finer layer. In general, the most 
significant production of DNAPL through any fine layer in these studies was actually 
upstream fi-om the source zone. 

Gradient injection to remove DNAPLs does not appear to provide significant 
benefit over step inputs. Override characteristics are not improved upon dramatically. 
Furthermore, interfacial tension between injected fluid and DNAPL decreases almost 
instantaneously compared to removal of DNAPL due to solubilization. Movement of high 
residuals of DNAPL down onto the finer layer occurs well before significant reduction of 
saturations due to solubilization. Here, solubilization of DNAPL becomes even less 
efficient due to essentially only one interface fi-om which to allow mass transfer. 

Entry pressure calculations using the physical and hydrogeo logic parameters of the 
chemical phases and media, respectively, predicted breakthrough of PCE into the finer 
media in excellent fashion. Breakthrough of PCE under typical ethanol flooding 
conditions (80% v/v) can generally be assumed to occur in homogeneous sand media 
when hf''^' < 0.35 cm. Calculations for a Beit Netofa clay estimated that approximately 

a half a meter worth of equilibrated PCE-type DNAPL would have to accumulate before 
entry into the clay pores under extreme cosolvent flooding conditions. 

Use of the partitioning alcohol t-butanol in the 2-D setup presented interesting 
qualitative observations. Significant swelling of the PCE resulted, especially for the 
40%v/v TBA cosolvent step-input. This swelling, caused by partitioning, and subsequent 



126 

delay in downward mobili2ation due to override, can help in avoiding breakthrough of 
DNAPL into finer zones, but can also lead to cosolvent becoming trapped on top of finer 
layers. This volume of cosolvent can produce increased tailing of contaminant removal 
and increase remediation times. Further study into its avoidance is warranted. 

There are quantitiative limitations to the theories discussed in this dissertation 
when one applies them to field situations. However, they are not that serious to prevent 
them fi-om being used in a more qualitative sense. In design of remediation technologies 
or strategies, such as cosolvent flushing, scientists or engineers naturally seek to optimize 
variable parameters. In the case of cosolvent flushing, these include the choice of a 
cosolvent "recipe" and mode of injection. "Exact" values of key parameters and 
relationships to other variables can be determined in the laboratory, but we seldom need 
that accuracy in the field application. Furthermore, addition of heterogeneity, dilution, 
and dispersion, do not allow exact predictions. The information presented in the previous 
pages is simply intended to aid in narrowing the many choices an engineer must face when 
removing a DNAPL from the subsurfece with cosolvent mixtures. 



APPENDIX A 
MOISTURE RELEASE CURVES FOR SAND MEDIA 



127 



128 




0.05 



0.1 



0.15 0.2 

Moisture Content 



0.25 



0.3 



0.35 



♦ Data — — Brooks&Corey ■ 



-vanGenuchten 



Figure A-1. Moisture release curve for Nos. 20-30 sand. 



■.^rt^ 



24000 

22000 

20000 

18000 

«• 16000 

>< 

u 

£ 14000 

3 

1^ 12000 

U. 

5 10000 ■ 

1 

O 8000 . 

eooo 

4000 

2000- 




0.125 0.15 0.175 

Pore Size (mm) 



0275 



- van Genuchten 



Brooks&Corey 



Figure A-2. Pore size frequency distribution of Nos. 20-30 sand. 



.i-'V^..: 



; r,- 



129 




0.05 0.1 0.15 0.2 0.25 

Moisture Content 



0.3 



♦ Data Brooks&Corey van Genuchten 



0.35 



0.4 



Figure A-3. Moisture release curve for Nos. 30-40 sand 



28000 



24000 

22000 

20000 

•C- 18000 

8* 16000 

s 

3,14000 

? 

U. 12000 

a 

S 10000 

K 8000 

6000 

4000 

2000 




008 01 0.12 

Pore Size (mm) 

- van Genuchten Brcwks&Coney | 



Figure A-4. Pore size frequency distribution of Nos. 30-40 sand 



0.05 



130 




0.1 0.15 0.2 0.25 0.3 

Moisture Content 

♦ Data Brooks&Corey van Genucbten 



Figure A-5. Moisture release curve for Nos. 40-50 sand. 



0.35 



0.4 



50000 



f 30000 



10000 




002 003 



004 0.05 O06 

Port Sizs (mm) 



I van Genuchten Brooks&Corey | 

Figure A-6. Pore size frequency distribution of Nos. 40-50 sand. 



008 009 



131 




0.05 



0.1 



0.15 0.2 0.25 

Moisture Content 



0.4 



♦ Data Brooks&Corey ■ 



- van Genuchten 



Figure A-7. Moisture release curve for Nos. 60-70 sand. 



180000-1 

IflOOflO J 




/ 


1«000 




/ 

/ 


uency f(r) 


r\ 


/ 

/ : 


Relative Freq 


/ 


Y 


«000 


// 


\ 


20000 




-^ 


^^ 



Pore Size (mm) 



-vanGenuclnten 



Broolts&Coney | 



Figure A-8. Pore size frequency distribution of Nos. 60-70 sand. 



132 




0.05 



0.1 



0.15 0.2 0.25 

Moisture Content 



♦ Data Brooks&Corey — ^van Genuchten | 



Figure A-9. Moisture release curve for Nos. 100-140 sand. 




002 0025 GOO 01 

Pore Size (mm) 

-van Genuchten Brooks&Corey I 



Figure A- 10. Pore size distribution of Nos. 100-140 sand. 



133 




0.05 



0.1 



0.15 0.2 

Moisture Content 



0.25 



0.35 



♦ Data Brooks&Corey vanGenuchten 



Figure A-6-1 1. Moisture release curve for wide distribution (#20-100) sand. 



3UUOoJ 


^ ; ■• 




■*■ / • 


25000- 


/^^~^\ / 


















> 


1 yx t 


o 




§20000- 


/ / \ 1 - ■ . '■^• 


3 




o- 


/ y' \ 


O 








u. 


/ / \ 


§15000- 


/ y \ 1 


■= 


/ j/ \ 1 


a 


/ y \ I 


o 


/ X \ 


K 


1/ \ 


10000- 


/ \ 


5000- 
0- 


^ 1^^^^-^ 



Pore Size (mm) 



- van Genuchten Brooks&Corey 



Figure A- 12. Pore size distribution of wide distribution (#20-100) sand. 



'? 



APPENDIX B 
TWO-DIMENSIONAL BOX SCHEMATICS 

The following pages contain the schematics drawn during each run discussed in the 
main body of the dissertation. 



134 



135 





Water Tracer 

Media : 

#20-30 Background 

#100-140 Finer Layer 



Dye front 

Dye trailing edge 

DNAPL Pool 




"■■^- 



*-• ^ . 



136 





Step Injection 100% alcohol 

Media : 

#20-30 Background 

#100-140 Finer Layer 



80% alcohol front 

DNAPL 

Shrinking DNAPL pool 




137 





Step Injection 80% alcohol 

Media : 

#20-30 Background 

#100-140 Finer Layer 



Dye front 
DNAPL Pool 




>--^ ':■▼■ •* '^..J-'-' ^ *■ 5 



- i 



138 





Water Tracer 

Media : 

#20-30 Background 

#60-70 Finer Layer 



Dye front 
DNAPL Pool 




139 





Step Injection 80% alcohol 

Media : 

#20-30 Background 

#60-70 Finer Layer 



80% alcohol front 

DNAPL 

Shrinking DNAPL pool 




140 





Gradient Injection 

Media : 

#20-30 Background 

#60-70 Finer Layer 



60% Alcohol front 

Dye trailing edge (74%) 

DNAPL 

Shrinking DNAPL Pool 




-..v-r--' -■' v^-^ 



141 





Water Tracer 

Media : 

#20-30 Background 

#40-50 Finer Layer 



Dye front 
DNAPL Pool 




142 





Step Injection 80% alcohol 

Media : 

#20-30 Background 

#40-50 Finer Layer 



80% alcohol front 

DNAPL 

Shrinking DNAPL pool 





143 





Water Tracer 

Media : 

#20-30 Background 

#30-40 Finer Layer 



Dye front 

Dye trailing edge 

DNAPL Pool 




144 





Step Injection 80% alcohol 

Media : 

#20-30 Background 

#30-40 Finer Layer 



80% alcohol front 

DNAPL 

Shrinking DNAPL pool 








145 





Step Injection 70% alcohol 

Media : 

#20-30 Background 

#30-40 Finer Layer 



70% alcohol front 

Dye trailing edge 

DNAPL 

Shrinking DNAPL pool 





146 





Multi-Step Injection 

Media : 

#20-30 Background 

#30-40 Finer Layer 



50% Alcohol front (IPV) 

60% Alcohol front (IPV) 

Dye trailing edge (50%) 

DNAPL 

Shrinking DNAPL Pool 




' » '^'.J'^'-'^^f,"^. 



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>■- 



BIOGRAPHICAL SKETCH 

Michael E. Van Valkenburg was bom at the U.S. Military Academy, West Point, 
New York, on October 23, 1963. He graduated from Amphitheater High School, Tucson, 
Arizona, in May 1981, and immediately attended Washington University in St. Louis. In 
May 1985, he was awarded a Bachelor of Science degree in chemical engineering. On the 
same day, he was commissioned a second lieutenant in the U.S. Air Force as a 
bioenvironmental engineer. His first duty location was at Williams Air Force Base (AFB), 
Mesa, Arizona, where he held the position of Chief, Bioenvironmental Engineering 
Services. In June 1997 he was transferred to Ellsworth AFB, Rapid City, South Dakota, 
where he was Chief, Environmental Protection and Monitoring Programs. Upon 
acceptance to an Air Force-sponsored graduate program, he attended South Dakota 
School of Mines and Technology from August 1989 to June 1991, and received a Master 
of Science degree in civil (environmental) engineering. Upon completion, he transferred 
to the United States Air Force Academy, Colorado Springs, Colorado, to become a 
member of the Department of Chemistry. Here he taught general, analytical, and 
environmental chemistry to undergraduate Air Force Cadets. From June 1994 to August 
1996 he was assigned to the Air Force Center for Environmental Excellence (AFCEE), 
Brooks AFB, Texas, where he was Deputy Chief, Pollution Prevention Programs Division. 
Upon acceptance of an Air Force sponsored Ph.D. program, he moved to Gainesville, 
Florida. Mike has a wife, Kim, and three children, Joseph, Lauryn, and Kelley. 



156 



I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is My adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 



[ichael D. Annable, Chair 
Associate Professor of 
Environmental Engineering Sciences 



I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosopl^ 





Jo^ph J. DelMio 

Tofessor of Environmental Engineering 
Sciences 



I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 



"Z/;V^47 



Robert T. Kennedy 
Professor of Chemistry 




I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




WiUiam R. Wise 

Associate Professor of Environmental 
Engineering Sciences 



I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 



■c^:/ ^-^ 




Kirk Hatfield 

Associate Professor of Civil 
Engineering 



This dissertation was submitted to the Graduate Faculty of the College of 
Engineering and to the Graduate School and was accepted as partial fiilfillment of the 
requirements for the degree of Doctor of Philosophy. r 

May 1999 .^^^^^/^ ^^^^i^^.^ 

Winfi-ed M. PhiUips 
Dean, College of Engineering 



M. J. Ohanian 

Dean, Graduate School 






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